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Old 10-20-2003, 09:22 PM   #1
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Exclamation HELP - Poor Pedal Pressure

Replaced the master cylinder today. On the last trip to MS I lost almost a half pint out of the old girl. Figured its time to make that old one a core return.

Put the new one on today, bench bled it, mounted it, and bled the lines (RR, LR, RF, LF) got out all the little bubbles). But Ė I still don't have decent pedal pressure. Any ideas? Should I buy/rent that brake bleeding tool? Anyone done this before?

Of course I leave tomorrow for a 2000 mile trip. Why do I decide to do this stuff? I know Ė because itís a quest to make everything better and it was only 50 bucks at NAPA. It was calling to me, taunting me, come on you can do it you can replace me right before you head outÖI dare you!

Seriously, Iím pretty hosed if I donít get this corrected ASAP.
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Old 10-20-2003, 09:30 PM   #2
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Steven,

How do the flex hoses from the frame to the calipers and rear axle look. These tend to get soft and will expand instead of pushing against the wheel cylinder. If they are original I would start there. It will require a re-bleed so it may be worth the new tool. This has been an issue on a few of my 10 year old cars.

I assume this pedal pressure is at rest with the engine running. I found that the age of the power steering fluid seems to have some effect too.
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Old 10-20-2003, 09:34 PM   #3
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A pressure bleeder is calling you, or a vacuum bleeder if you have a helper.

I don't think you could get all the air out with the master cylinder. When the brake lines go inside the cross members they go up for clearance and that is where the air stays. There just isn't enough displacement in the master cylinder to move it through so big a system.

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Old 10-20-2003, 10:30 PM   #4
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Put the new one on today, bench bled it, mounted it, and bled the lines (RR, LR, RF, LF) got out all the little bubbles). But Ė I still don't have decent pedal pressure. Any ideas? Should I buy/rent that brake bleeding tool? Anyone done this before?

Steven, do you mean it takes too much effort to mash the brake pedal to make the coach stop? Or is the pedal all the way to the floor, and you are still rolling?
To make sure you get all the air out of the master cylinder, you have to bench bleed it. That is, you have to fill the thing with brake fluid, and put those plastic caps on the outlets, and the rubber hoses on them, and run the hoses up into the reservoir, making sure the ends of the hoses are submerged. Then, push the plunger in the end of the master cylinder until no more air bubbles come out. Then install (or re-install) the Master cylinder, and bleed the rest of the system like you described.
If it takes too much effort to push the pedal, you could have a bad booster. If it is a hydro-boost type, running from the power steering, check and make sure the power steering pump is full of fluid. If you have the old vacuum type booster, check to make sure the vacuum hose is connected to the booster.
Good luck, and happy motor-homing!
Terry
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Old 10-21-2003, 08:19 AM   #5
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Thanks for all the replies. Just to answer everyone's questions:

1. Pedal goes to the floor with little resistance both engine on and off
2. Before I replaced the cyl it had decent pressure about 2" into the pedal
3. Vac pump is working (or was)
4. You can "pump up" pressure with multiple strokes.
5. The brakes worked well before the replacement...good pressure, good stopping, minimal pedal effort.
6. New front calipers and flex lines, checked the rears when replacing the shocks and the "looked" fine but since things work before I don't think its the rear lines.

I'm beginning to think I did not bench bleed the cylinder correctly. Getting no air bubble from each caliper but way to soft on the pedal. Must have air trapped somewhere in there.

Going to NAPA/AutoZone to get the tool today. I'll keep you posted

Steven
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Old 10-21-2003, 08:21 AM   #6
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I think you just need to do some more bleeding.
Its best to have three people on hand to do the manual bleeding. Pumper, filler and bleeder.
Using the air pressured bleeding machine still is best accomplished with 2 people. Filler and operator.
I assume you very carefully compared the two master cylinders and took great care in proper seating?
Be nice to yourself and put a quality pair of vicegrips on the line mounting nuts at the master and give it the extra torque. If you want to know why, ask me why I purchased some good vicegrips in Taos, NM after descending from a 9k pass.
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Old 10-21-2003, 08:44 AM   #7
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GM often is problematic to bleed. Many times it's air trapped in the proportioning valve(PV). Find the PV and see how it's mounted. The rear circuit seems to be very prone to trapping air. Th inlet from the master cylinder is on the top and the outpout is on the low end on most GM PV's.

WHat I have found that helps is to make sure the master is full of fluid. Loosen the inlet lines on the PV till it leaks at a pretty good rate. Tap the PV with a wrench as it's leaking. The master will gravity bleed so no air will work back up to it. Tapping the PV will cause the air to dislodge and hopefully come out with the fluid. Retighten the line and try breeding again.


ANother thing to look at is how the lies are routed. The RR-LF Bleed sequence may not work well. One of the front outlets is higher then the other. On my GM trucks that is the Left front. Logic tells you that air rises to the top in fluid. Logic would be bleed the top line first to get the air out fastest. On my GM truck Bleed rear first (side doesn't matter because on mine it's a single line back that "T's" on the axle. Bleed the Left front then the right front.


The master HAS to have rear pressure to get the front to bleed out. The piston for the rear has mechanical connection to the peddle but when properly bleed the master traps fluid between the rear piston and the front piston. That fluid is what actually pushes the front piston. Now if the rear circuit does loose pressure the rear piston will botom out against the front piston but the peddle travel will be excessive.

I have had mixed luck with the Suction bleed kits you can pick up. If the system is fairly air free they work but if Air is in the PV it will suck the PV shut.

My method is to try to gravity bleed the system first. Simlyp open the bleed valves and let the fluid drain. for a couple minutes eache wheel. I usualy try to both rears at the same time because of the T at the axle. Then I trymy suction bleed if that has failed to work alone. Then I grab my wife or my 11 year old and do the pump method.

A pro mechanic friend of mine has a trick he swears by on stuborn systems. He has some junk yard master cyl caps he has put some fittings on. This allows him to put a slight pressure and force fluid through the system (10psi max 3-4 is usualy enough). THis allows him to by himself go around and open each bleed valve to perge the air. He has also had luck by putting a slight vacuum (about 10HG) on the sytem to draw the air that may be trapped in high spots back up to the master. Usualy leaves it under a vacuum state for 2-3 hours.

I haven't tried either method but both make sense. I have used Vac tricks in the printing buisness to remove air bubles from inks and casting resins we had to do for parts. Works well in that application and seems like it should work for this application.
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Old 10-21-2003, 11:54 AM   #8
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Quote:
My method is to try to gravity bleed the system first. Simlyp open the bleed valves and let the fluid drain. for a couple minutes eache wheel. I usualy try to both rears at the same time because of the T at the axle. Then I trymy suction bleed if that has failed to work alone. Then I grab my wife or my 11 year old and do the pump method.

A pro mechanic friend of mine has a trick he swears by on stuborn systems. He has some junk yard master cyl caps he has put some fittings on. This allows him to put a slight pressure and force fluid through the system (10psi max 3-4 is usualy enough).
Too big of a system to let gravity suck the air out. There is probably 25 ft. of line to the rear axle. Every crossmember has an offset up for clearance. The air is going to get into these offsets and stay. Your mechanic buddy is using a pressure bleeder, that is the best way to bleed any brake system. Once the mc is bled it forces the air from the system, start to finish and flushes the old fluid out.

John
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Old 10-21-2003, 12:22 PM   #9
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Talked to the service manager at AC Brake here in town (commerical truck brake shop)....said that I should not need to pressure bleed a GM P30 Chassis. Suggested gravitity bleading the front circuit for 30 minutes...then bleeding each caliper again.

Getting ready to try it along with all the other good forum tips so far.
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Old 10-28-2003, 04:35 PM   #10
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Finally back

OK...Just got back from a trip to NE. The gravity bit worked great. Had to really stomp on the brakes in NY when a driver pulled out in front of me...no problems and no more leaks.

Thanks to all for the tips.
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Old 10-28-2003, 07:13 PM   #11
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Stomping down

Great - Gives me something to look forward to.

Who knows how long the brake fluid has been in the Xilver Xeppelin, and I do not feel real comfortable with the 18 year old rubber hoses connecting everything.

One of the close proximity projects will be to replace the flexible parts of the brake system, inspect everything one more time, and ensure that the fluid is properly changed.

I was reviewing the old threads on trailer brakes, and learned a lot about the system on the tag axel.
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