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Old 03-20-2011, 05:23 PM   #61
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Thank you both. I am going to drive out there tomorrow to check in. If I understand they are locking up on them. I will post tomorrow after I talk with them. Thank you again! I just want the damn thing back. Haha. I miss her.
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Old 03-22-2011, 08:15 PM   #62
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Yep. It needed to be shimmed. But let me tell you tag axle, breaks now work, all new lines, calipers, pads. Stops great!
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Old 03-24-2017, 03:03 PM   #63
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I don't know who is still viewing this thread but I need advice. Recently replaced hydroboost and master cylinder on the 1991 350LE. Unfortunately before returning the old hydroboost for reman credit, I failed to remove the push-rod connecting the hydroboost to the master cylinder. By the time I realized my mistake, my return was shipped of to the remanufacturer. Not realizing that the push-rod length was critical, I was able to determine that the appropriate push-rod length for this set-up was 3.442" (GM part number 00377397, Bendix part number 129959)..both NLA. Found two replacements at the local wreckers (3.34" and 3.14"). Based on the GM length spec, installed the one measuring 3.34". Brakes worked great until used a few times..some time in "panic mode" to ensure all was well. Then things went wrong. The brakes started to lock-up (mostly front) until the newly rebuilt 454 could no longer move the MH! Smell of burning brake material and very hot disks particularly on the front. Waited for about an hour and, was able to drive home with normal very gentle braking.
Everything I read tells me the problem is that the push-rod I installed is too long and lot allowing the MC to bleed-off brake pressure. so, if that is the case, should I install the 3.14" version or am I chasing another problem. Help please!
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Old 03-24-2017, 04:20 PM   #64
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Hi Chris,

Sounds like a right PITA. I had a quick scan (as I am sure you have) and can't see any for sale. Do you think it's possible that the rod was available as part of a kit? Something like this, although don't think this is the one:

http://www.oldsobsolete.com/products...-nos-14055058/

This thread might shed some light
http://www.4btswaps.com/forum/archiv...p/t-13890.html

Otherwise, try the guys on the Chevytruck forums. They might be able to turn up some leads.

The Light Truck Unit service and repair manuals for 1991 (I posted link in post 94 on the sticky) will tell more about the breakdown and fault finding for the master cylinder and hydro-boost, although you already look to have good information, so maybe not of help.

I do intend to replace the master cylinder and possibly the hydro-boost on my 1991 250, although that is unlikely to be for at least six weeks, possibly more. I can look at it all when I get it taken apart and get all the relevant part numbers/measurements, although appreciate that this is some way off. PM me if interested.

Good luck

Nick
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Old 03-24-2017, 07:07 PM   #65
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Do you think it was possible you boiled your brake fluid during you test panic stops? If the fluid is old it probably contains some moisture. I had this happen to me in some rolling hills with my 345. Once it starts it seems to run away. Brakes dragging and generating more heat. I found myself giving it more gas until I realized something was wrong. Then it took both feet on the brake pedal and the emergency brake as well to bring it to a stop. White knuckles and smoking brakes all around. After cooling off for an hour, the trip home was fine. Needless to say I replaced all pads, calipers, fluid, and hydro boost that winter. But in hindsight I believe the fluid was the cause. Just some food for thought.
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Old 03-24-2017, 09:19 PM   #66
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All new components and fluid

Quote:
Originally Posted by mehes View Post
Do you think it was possible you boiled your brake fluid during you test panic stops? If the fluid is old it probably contains some moisture. I had this happen to me in some rolling hills with my 345. Once it starts it seems to run away. Brakes dragging and generating more heat. I found myself giving it more gas until I realized something was wrong. Then it took both feet on the brake pedal and the emergency brake as well to bring it to a stop. White knuckles and smoking brakes all around. After cooling off for an hour, the trip home was fine. Needless to say I replaced all pads, calipers, fluid, and hydro boost that winter. But in hindsight I believe the fluid was the cause. Just some food for thought.
Thanks for your suggestion however, the brake fluid was very fresh as was the hydroboost and master cylinder. We will be doing more testing this weekend to absolutely establish the cause of brake locking (also to closely inspect the caliper pistons in case). The reason for posting on this thread was to see if anyone had physically measured the length of the push-rod as some point so we could compare. Thanks again and glad your situation was resolved.
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Old 03-24-2017, 09:34 PM   #67
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I think you need to read this link. It explains a lot of how the hydroboost system can affect the brakes and the symptoms.

Part of the link provided.

5. If brakes are self applying and pedal is free, check for obstruction in the return line or a kinked connection between hydo-boost and pump reservoir. If obstruction or kink is found, go to step 6, otherwise go to step 7.

http://www.brakeandfrontend.com/oper...ssist-systems/

cheers
Tony
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Old 03-24-2017, 09:34 PM   #68
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Thanks for response

Quote:
Originally Posted by Boom Sounds View Post
Hi Chris,

Sounds like a right PITA. I had a quick scan (as I am sure you have) and can't see any for sale. Do you think it's possible that the rod was available as part of a kit? Something like this, although don't think this is the one:

http://www.oldsobsolete.com/products...-nos-14055058/

This thread might shed some light
http://www.4btswaps.com/forum/archiv...p/t-13890.html

Otherwise, try the guys on the Chevytruck forums. They might be able to turn up some leads.

The Light Truck Unit service and repair manuals for 1991 (I posted link in post 94 on the sticky) will tell more about the breakdown and fault finding for the master cylinder and hydro-boost, although you already look to have good information, so maybe not of help.

I do intend to replace the master cylinder and possibly the hydro-boost on my 1991 250, although that is unlikely to be for at least six weeks, possibly more. I can look at it all when I get it taken apart and get all the relevant part numbers/measurements, although appreciate that this is some way off. PM me if interested.

Good luck

Nick
Nick..I already owed you a reply to your PM and will do so after "le crise".
The issue here seems to be that, although a standard exists for hydroboost length of push "rest and travel" of the piston, no standard exists for length of travel of the master cylinder and indeed the length of the mastger cylinder itself. This is the reason why standard parts were discontinued and the re-manufactured hydroboost carries the warning to remove the push-rod, spring, washer and retainer BEFORE returning the old hydroboost. Unfortunately, this instruction is "buried" in the box and other instructions which was my "undoing"
You are on the right track with your GM/Chevy forum comment as that is where I was able to determine the length the push-rod plays in brake performance (too short, no brakes, too long locks brakes). I have also found the procedure for measuring the correct depth of the push-rod to ensure adequate return of the master cylinder piston which is what we will be doing next week when I remove the master cylinder for the second time. My reason for posting this issue is in the hope that (before you carry-out your refurbish) someone can confirm the correct push-rod length which I think is 3.14"
In any event, make sure you retain your old hydrobooster and bits until you install and check mc interface and operation.
I'll let you know what happens. Thanks Nick for your input.
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Old 03-24-2017, 09:51 PM   #69
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Disappointment with article

Quote:
Originally Posted by Isuzusweet View Post
I think you need to read this link. It explains a lot of how the hydroboost system can affect the brakes and the symptoms.

Part of the link provided.

5. If brakes are self applying and pedal is free, check for obstruction in the return line or a kinked connection between hydo-boost and pump reservoir. If obstruction or kink is found, go to step 6, otherwise go to step 7.

http://www.brakeandfrontend.com/oper...ssist-systems/

cheers
Tony
Tony my friend..thanks for the above link however, I had already found this and thought it might be useful. But..although below figure 11, the author indicates dealing with 4 problems (noise, pedal return, over sensitive braking and self-applying brakes), there is no specific reference to the self-applying brakes and in particular the relationship of the push-rod length being related to "self-applying brakes". Great article if one is having problems with hydroboost operation but mine is working fine..the push-rod extends on brake pedal operation and retracts when the pedal is released. The problem I'm having is determining the return travel of the push-rod that allows the master cylinder piston to release brake pressure after the pedal is released...thnaks again..will keep al informed of any solution that we find!
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Old 03-24-2017, 10:37 PM   #70
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Having been through a lot of this exact grief with my 310, I can tell you that yes, the rod length is critical, and you can put a shim between the MC and hydroboost to correct for it. It also varies from unit to unit, so be very cautious about some magic dimension that might or might not apply to your unit.

In my case, the lockup problem was mostly the result of very fine rust particles collecting in the MC. When applying the brakes, that pressure was enough to actuate the calipers. The spring in the calipers (which also get rusty and drag) was not enough to push back through those orifices to get the fluid back into the MC. Is the fluid in the MC clear or rusty?
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Old 03-25-2017, 09:49 AM   #71
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Agree

Quote:
Originally Posted by dljosephson View Post
Having been through a lot of this exact grief with my 310, I can tell you that yes, the rod length is critical, and you can put a shim between the MC and hydroboost to correct for it. It also varies from unit to unit, so be very cautious about some magic dimension that might or might not apply to your unit.

In my case, the lockup problem was mostly the result of very fine rust particles collecting in the MC. When applying the brakes, that pressure was enough to actuate the calipers. The spring in the calipers (which also get rusty and drag) was not enough to push back through those orifices to get the fluid back into the MC. Is the fluid in the MC clear or rusty?
Appreciate for your comments..absolutely agree that the rod length is critical..the issue I am having is determining the correct rod length that (a) applies brakes and (b) retract sufficiently to allow the brakes to de-pressurize. How did you determine the number/width of shims required to achieve correct rod actuation length? As I stated earlier, there is no such thing as a "standard length" it seems.
MC is new, brake pads and rotors were replaced last year and fluid is clear as new. We will however have a close look at the caliper action but as the lock-up is on all four disk brakes..mmmm. Thanks again.
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Old 03-25-2017, 04:21 PM   #72
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Chris,

Not sure if you have already stumbled across this, but might be of some use

http://www.machinesoflovinggrace.net...Cylinders.html

I assume you do have JB8 RPO variant (which is my version) from your VIN and nothing different.
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Old 03-25-2017, 05:07 PM   #73
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Might also be worth calling AC Delco. In my experience, they seem to have additional information on parts, which might in this case, include the precise dimensions of the parts you reference.
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Old 03-26-2017, 12:34 AM   #74
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MC locking brakes.

As I have just done my 345's brakes all around must still have air posibly proportional valve?, anyway I do remember reading this regarding locked brakes..slacken the master cylinder to hydoboost bolts enough to compensate for rod length discrepancy at least to get you home. Also another experience was as my pads had worn down alot this caused the fluid level to go down in the MC which causes a vacume to form in the chamber that causing a very low soft brake peddle, till we popped the lid and got rid off that vacume.
Lovely Machines really.
Rus
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Old 03-26-2017, 03:42 AM   #75
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One final thought on this topic.

My gut feel here is that the push rod length difference might be caused by slight differences in the original set up. In other words, your coach either came with the Delco-Moraine or the Bendix MC. They now show as interchangeable parts, but in fact they are not. They will fit, as will many other MCs, but the precise output rod length is absolutely critical to proper operation. This would be consistent with posts I have read on the topic and people either tweaking the rod length or adding some kind of spacer to adjust for tolerances.

I think the stock MC units were different in colour, so even if you have chucked it away, you might be able to match the original with a rod from a breakers, based on the colour of the original MC. Good luck Chris.

I have already purchased an AC Delco MC and repair kit. It will be interesting to see what comes off my coach. I shall take pictures and put something up on the forum and measure the output rod when I get it all replaced.

Nick

NB. Note to self, start this job well in advance of the school and summer holiday.
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Old 03-26-2017, 05:28 AM   #76
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Quote:
Originally Posted by chriscraw View Post
I don't know who is still viewing this thread but I need advice. Recently replaced hydroboost and master cylinder on the 1991 350LE. Unfortunately before returning the old hydroboost for reman credit, I failed to remove the push-rod connecting the hydroboost to the master cylinder. By the time I realized my mistake, my return was shipped of to the remanufacturer. Not realizing that the push-rod length was critical, I was able to determine that the appropriate push-rod length for this set-up was 3.442" (GM part number 00377397, Bendix part number 129959)..both NLA. Found two replacements at the local wreckers (3.34" and 3.14"). Based on the GM length spec, installed the one measuring 3.34". Brakes worked great until used a few times..some time in "panic mode" to ensure all was well. Then things went wrong. The brakes started to lock-up (mostly front) until the newly rebuilt 454 could no longer move the MH! Smell of burning brake material and very hot disks particularly on the front. Waited for about an hour and, was able to drive home with normal very gentle braking.
Everything I read tells me the problem is that the push-rod I installed is too long and lot allowing the MC to bleed-off brake pressure. so, if that is the case, should I install the 3.14" version or am I chasing another problem. Help please!

http://www.airforums.com/forums/f311...ml#post1927046
Slowly and evenly unbolting the master cyl and using a feeler gauge, when it is pre loaded, will give you a start point for correcting the length of the pushrod.

2 components can cause your problem, in your case its probably the push rod. The other is the master cylinder. The distance of the red bar (at the black arrow) needs to match your existing system.
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Old 03-26-2017, 08:30 AM   #77
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If you identify the difference using the feeler gauge, you can get one custom made for 30 bucks. There is a post about it somewhere. I think it was from Tony.


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Old 03-26-2017, 02:34 PM   #78
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Lovely Machines really!

Quote:
Originally Posted by Russell Col View Post
As I have just done my 345's brakes all around must still have air posibly proportional valve?, anyway I do remember reading this regarding locked brakes..slacken the master cylinder to hydoboost bolts enough to compensate for rod length discrepancy at least to get you home. Also another experience was as my pads had worn down alot this caused the fluid level to go down in the MC which causes a vacume to form in the chamber that causing a very low soft brake peddle, till we popped the lid and got rid off that vacume.
Lovely Machines really.
Rus
Said like a true Kiwi and absolutely correct except when it comes to troubleshooting this bloody rod length issue. Just letting the brakes bleed back allowed me to get back to base. Another attempt after a few tentative steps..same lock-up condition. Will post more after checking push-rod length.
BTW new pads and rotors Thanks
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Old 03-26-2017, 02:38 PM   #79
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Advance Warning!

Quote:
Originally Posted by Boom Sounds View Post
One final thought on this topic.

My gut feel here is that the push rod length difference might be caused by slight differences in the original set up. In other words, your coach either came with the Delco-Moraine or the Bendix MC. They now show as interchangeable parts, but in fact they are not. They will fit, as will many other MCs, but the precise output rod length is absolutely critical to proper operation. This would be consistent with posts I have read on the topic and people either tweaking the rod length or adding some kind of spacer to adjust for tolerances.

I think the stock MC units were different in colour, so even if you have chucked it away, you might be able to match the original with a rod from a breakers, based on the colour of the original MC. Good luck Chris.

I have already purchased an AC Delco MC and repair kit. It will be interesting to see what comes off my coach. I shall take pictures and put something up on the forum and measure the output rod when I get it all replaced.

Nick

NB. Note to self, start this job well in advance of the school and summer holiday.
Good idea given my tribulations...Nick I believe your "gut feel" is correct as the hydroboost unit and MC's are indeed not interchangeable hence the need to have a way of determining the correct rod length when fitting both these re-manufactured parts together. so lots of "rod length tweaking" next week..will advise result of put a lightly used 350LE up for sale "as is"!
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Old 03-26-2017, 02:43 PM   #80
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Quote:
Originally Posted by Boom Sounds View Post
If you identify the difference using the feeler gauge, you can get one custom made for 30 bucks. There is a post about it somewhere. I think it was from Tony.


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Nick..correct..Current price for custom rod is $US35 with a 2 week delivery...issue really is determining the exact required length..Tallon will make to any length.
http://www.tallonhydraulics.com/stor...ex&cPath=40_16
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