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Old 04-21-2009, 11:31 PM   #29
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Paul I cant tell you exactly what a failing hydroboost feels like I can tell you that mine stops very well with everything replaced however make sure you shimmed that master cylender otherwise you will be driving down the road and the brakes will apply themselves without input... That is not fun..
Justin
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Old 04-22-2009, 12:00 AM   #30
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Originally Posted by pwhittle View Post
I just replaced my master cylinder to address a leak, and I was hoping my brake pedal would have improved. However, it remains the same. We flushed the fluid out when we replaced teh MC.

My pedal travels a long way before the brakes start to apply, then it still seems soft.

I will flush out the power steering fluid and check the pedal to hydroboost linkage to see if that helps.

What do your brake pedals feel like? Mine are not very confidence inspiring, but I want to make sure my expectations are set correctly.

What are the symptoms of a failing HydroBoost?
Paul
Hi, a hydroboost won't have a hard pedal, especially while stopped. While driving will your vehicle stop like it should? If you don't have the correct Master Cylinder replacement, it won't work right. Many parts look the same or similar, but are different. If for example your new Master Cylinder has smaller bore/pistons than the original, you will have more travel to push the same amount of brake fluid. When a vacuum brake booster fails, it is very hard to stop the vehicle; When a Hydroboost fails, it is nearly impossible to push the pedal hard enough to barely slow down. [will feel like slideing on ice]
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Old 04-22-2009, 12:30 AM   #31
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The replacement was a Cardone rebuilt unit and had the same casting numbers and dimensions as the one I took off including the bore size. There were no shims on the one I took off or the one I put on.

Justin, what is the basis for the shimming and what dimensions are we trying to achieve?

It stops OK, but it does not have a confidence inspiring amount of pedal travel left, but that is what it was like before the master cylinder replacement.

Let me get the power steering flush and the pedal to hydoboost linkage checked first and then see what I get.

Paul
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Old 04-22-2009, 08:46 AM   #32
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never mind. duplicate info.
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Old 04-22-2009, 05:58 PM   #33
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Paul,

Had the same sort of problem on our 84 310 after having the hydraboost and the master cylinder replaced at a big truck service shop. Took them a month to get it right. They replaced the mc twice and the booster 3 times before they finally realized the whole problem was air in the lines. They ended up buying a pressure bleeder to try and fix it and that still didn't get the job done. After finally trying it manually, with very slow pedal pushes and pumping about 4 gallons of fluid through the system they got the brakes to work right. The pedal now is very firm and probably goes halfway to the floor and stays firm even when stopped. I'm a lot more comfortable with the brakes now.

One other thing we had done was to replace all of the rubber hoses. After 25 years the old ones were just to brittle and I was worried about driving it until we got them replaced.

Hope this helps!

Brad
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Old 05-26-2009, 07:45 PM   #34
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I hope that folks are still reviewing this thread -- I have a similar problem with my 83 310, which like the 345 has disk brakes all around. It has a Mico Lever-Lock instead of a parking brake. I drove the rig home from Seattle, about 850 miles, a couple of months ago with no problems, using the Lever-Lock as a parking brake. It would apply and release properly each time.

The rear brakes started dragging. After a few miles they would lock up almost entirely (full throttle for 5 mph). Opening a bleeder, even after the rig was stopped for 30 minutes as I thought about what to try, resulted in a squirt of brake fluid - there was a lot of pressure still in the system. Everything OK for another few miles. Only the rear brakes were being applied (still hot while the fronts were cool).

We tried pulling both rear calipers. The slides were dry and a little rusty (we cleaned and lubricated anyway) but nothing too bad. The cylinders moved freely and retracted as they should.

MC is certainly a possibility. But why would it fail in this way just out of the blue? Maybe the booster? How would I tell?
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Old 05-26-2009, 08:27 PM   #35
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Originally Posted by dljosephson View Post
I hope that folks are still reviewing this thread -- I have a similar problem with my 83 310, which like the 345 has disk brakes all around. It has a Mico Lever-Lock instead of a parking brake. I drove the rig home from Seattle, about 850 miles, a couple of months ago with no problems, using the Lever-Lock as a parking brake. It would apply and release properly each time.

The rear brakes started dragging. After a few miles they would lock up almost entirely (full throttle for 5 mph). Opening a bleeder, even after the rig was stopped for 30 minutes as I thought about what to try, resulted in a squirt of brake fluid - there was a lot of pressure still in the system. Everything OK for another few miles. Only the rear brakes were being applied (still hot while the fronts were cool).

We tried pulling both rear calipers. The slides were dry and a little rusty (we cleaned and lubricated anyway) but nothing too bad. The cylinders moved freely and retracted as they should.

MC is certainly a possibility. But why would it fail in this way just out of the blue? Maybe the booster? How would I tell?
It could be the master cylinder, or the brake hose to the rear axle has collapsed internally. It will let pressure in, but not back out. Tracking it down is easy, but messy. Apply and release the brakes several times, then crack the rear brake line line at the master cylinder. If there is pressure there, the master cylinder is likely the culprit. If there is no pressure (no squirting fluid), move to the rear brake hose, above the axle. Try the same thing there, crack the line and see if fluid squirts out. It's possible the little adjustable rod for the brake pedal/master cylinder needs to be adjusted inward a bit, but if no one has touched it, or the booster or master cylinder, it is unlikely.
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Old 05-26-2009, 08:35 PM   #36
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Quote:
Originally Posted by dljosephson View Post
I hope that folks are still reviewing this thread -- I have a similar problem with my 83 310, which like the 345 has disk brakes all around. It has a Mico Lever-Lock instead of a parking brake. I drove the rig home from Seattle, about 850 miles, a couple of months ago with no problems, using the Lever-Lock as a parking brake.
What is a "Mico Lever-Lock"? I've never had brake problems, except on 10% grades where I wished for a Jake Brake.
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Old 05-26-2009, 08:42 PM   #37
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It could be the master cylinder, or the brake hose to the rear axle has collapsed internally. It will let pressure in, but not back out. Tracking it down is easy, but messy. Apply and release the brakes several times, then crack the rear brake line line at the master cylinder. If there is pressure there, the master cylinder is likely the culprit. If there is no pressure (no squirting fluid), move to the rear brake hose, above the axle. Try the same thing there, crack the line and see if fluid squirts out. It's possible the little adjustable rod for the brake pedal/master cylinder needs to be adjusted inward a bit, but if no one has touched it, or the booster or master cylinder, it is unlikely.
Terry, since this is a 310 its likely going to have two rubber hoses for the rear brakes, one for each rear caliper. While its possible I would be surprised if they both failed at the same time.

I'm inclined to think its probably the master cylinder or the booster. I seem to remember a thread where the rod between the master cylinder and the booster has to be just right or the brakes won't work right. Of course that doesn't explain why it would all of a sudden fail. Needless to say this one is a tough one to diagnose.

Brad
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Old 05-26-2009, 08:47 PM   #38
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What is a "Mico Lever-Lock"? I've never had brake problems, except on 10% grades where I wished for a Jake Brake.
IIRC, it's a lever that blocks the brake lines, and keeps fluid pressure from dropping while parked. I have seen one fail, but it was an "almost no movement possible" failure.

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Terry, since this is a 310 its likely going to have two rubber hoses for the rear brakes, one for each rear caliper. While its possible I would be surprised if they both failed at the same time.

I'm inclined to think its probably the master cylinder or the booster. I seem to remember a thread where the rod between the master cylinder and the booster has to be just right or the brakes won't work right. Of course that doesn't explain why it would all of a sudden fail. Needless to say this one is a tough one to diagnose.

Brad
I wasn't sure if it was one hose or two, but I'm thinking it's probably master cylinder (which is why I suggested starting there), or that Lev'r Lock.
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Old 05-26-2009, 09:04 PM   #39
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Was the lever lock a add-on? I have a simple parking brake off the overdrive shaft.
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Old 05-26-2009, 09:12 PM   #40
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Was the lever lock a add-on? I have a simple parking brake off the overdrive shaft.
It was an add-on, usually installed on manual transmission vehicles with lots of in-and-out of the cab, such as delivery and tow trucks.
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Old 05-26-2009, 11:30 PM   #41
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since this is a 310 its likely going to have two rubber hoses for the rear brakes, one for each rear caliper. While its possible I would be surprised if they both failed at the same time.
how does the line run to the rear axle? does it go from a single line along the frame to a rubber line on the axle and then split to two steel lines to the calipers?

check for a squashed steel line too.
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Old 05-27-2009, 01:20 AM   #42
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To Mike Leary: The Mico Levr-Lock is a manually controllable check valve that allows you to step on the brakes, shift a lever, and hold the brake pressure just as if you were still stepping on the brakes. This 310 had no parking brake (maybe it had a drive line brake before the Gear Vendors was added, but there isn't one now). I first used them in fire trucks that had Allison automatic transmissions (which have no Park).

I see that the Mico instructions say not to use these things for more than an hour. Maybe that's the problem, I used it at least overnight, maybe these hoses don't like to have pressure for that long. I think I have enough info to chase this all down now... arrgh.

To rchinny and overlander63, thanks for the troubleshooting points. Aside from this problem, the brakes work fine -- they are certainly more powerful than the Isuzu or 11,000 pounds empty going downhill. There is one steel line to the back, a cube-shaped tee and steel lines to the calipers, with short rubber sections. The mechanic who came out to look also mentioned this possibility that the rubber lines had deteriorated to the point where they would act like checkvalves ... we'll see.

Now I wonder why Bendix/GM disk brake calipers have metric bleeder screws !?
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