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daveDC · 10-02-2020, 07:54 PM

help i am at wits end. i have a 1999 dynamax sitting on a p30 gm chassis.my rear drum brakes are self applying while driving. i have replaced the brake shoes,wheel cylinders, rubber hose to the rear end, ive flushed and refilled the brake fluid several times.i have the parking brake disconected. i drive 20 miles and the rear brakes are hot. any input would be appreciated do i need to replace the master cylinder or booster? or both?

arcaguy · 10-02-2020, 08:20 PM

Are the brakes adjusted properly? If you can turn the brake drums by hand when the wheels are off the the ground they probably are. It's been a long time since I dealt with drum brakes and the old memory is getting foggy. Sorry I couldn't be any more help.

NvGuy · 10-02-2020, 08:36 PM

Short answer: look into replacing the booster.
I had a Dodge truck in which the booster would pull the brake pedal down, lightly applying the brakes when cold. once it warmed up, the brakes worked Ok.

twinboat · 10-02-2020, 09:26 PM

Pull upon the brake pedal after a stop and see if that helps.
If it does, the linkage and bellcrank is binding.

Otherwise, the slight gap between the booster rod and master cylinder is to close.
If you don't let the master cylinder piston return fully, the fluid can't return from the wheels.

daveDC · 10-03-2020, 05:18 AM

if i jack it up i can spin the wheels freely, i greased the linkage and bellcrank the move freely. i am going to replace the booster and master cylinder. $$$$$$ i will post results when i am done.

LETMGROW · 10-03-2020, 05:48 AM

In your driveway, start the engine and press the brake pedal several times. Shut the engine off, crawl under the coach and open a rear brake bleeder and watch the fluid run out. If you get a spurt of fluid under pressure come out you will know for sure something is holding pressure on the system.
If you do see pressure close the bleeder and repeat starting the engine and pressing the pedal. Shut the engine down and loosen the bleeder on the cylinder or the nut holding the brake line leading to the rear brakes. If you get a spurt of fluid you will know fluid isn't returning into the cylinder. If you don't then there is a restriction somewhere between the wheel cylinders and the master cylinder.
If you suspect the master cylinder is at fault I suggest replacing the master cylinder first or taking the master cylinder off and disassemble it and give it a thorough cleaning being sure the ports are free of dirt obstructing the fluid returning when the pedal is released. Rebuild kits should be readily available. or you can buy a new or rebuilt master cylinder.

LETMGROW · 10-03-2020, 05:23 PM

Quote:

Originally Posted by LETMGROW

In your driveway, start the engine and press the brake pedal several times. Shut the engine off, crawl under the coach and open a rear brake bleeder and watch the fluid run out. If you get a spurt of fluid under pressure come out you will know for sure something is holding pressure on the system.
If you do see pressure close the bleeder and repeat starting the engine and pressing the pedal. Shut the engine down and loosen the bleeder on the cylinder or the nut holding the brake line leading to the rear brakes. If you get a spurt of fluid you will know fluid isn't returning into the cylinder. If you don't then there is a restriction somewhere between the wheel cylinders and the master cylinder.
If you suspect the master cylinder is at fault I suggest replacing the master cylinder first or taking the master cylinder off and disassemble it and give it a thorough cleaning being sure the ports are free of dirt obstructing the fluid returning when the pedal is released. Rebuild kits should be readily available. or you can buy a new or rebuilt master cylinder.

My apologies.
In my second step I meant to say loosen the bleeder on the 'MASTER' cylinder or the line on the master cylinder leading to the rear brakes. Due to different design variations not all master cylinders have bleeders on them.
One further step I should have included. Before you condemn the master cylinder loosen the bolts or nuts holding the master cylinder to the brake booster and let the master cylinder move ahead away from the booster slightly. Then try pressing the brakes and check again for pressure. If you don't have pressure on the system the booster is your problem. It would be holding pressure against the master cylinder.

RickNC · 10-04-2020, 01:41 PM

Letmgrow gave you good advice on checking for line pressure, I have the same brake setup as you on a 1995 P30. A few other things to consider;

Did you replace the hardware and return springs on the rear shoes when you replaced them? There is a few psi of residual pressure in the rear brakes to keep the brake cylinder pistons and push rods in contact with the shoes. If the springs are stretched and weak this can contribute to the problem. Make sure the right spring is in the right position. Make sure the leading and trailing brake shoe are in the correct locations.

Are your drums out of round? The drum brakes are self energizing any drag from out of round drums amplifies a bit and will build heat.

Not a common problem but the brake combination valve (AKA proportioning valve) that maintains the residual pressure in the rear drum brakes and balance the pressure between the front calipers and rear wheel cylinder can malfunction. The combination valve is about 12 “ to the rear of the MC inside of the frame rail.

There is no residual valve in the master cylinder on the P30. I can only guess that rear piston in the MC hanging up on return could cause this problem.

As for the booster to MC rod clearance. I doubt that one because my logic would say if the clearance is too tight both front and rear brakes should drag. But I could be wrong. If it were my bus I would loosen it like LetMGrow says but I would insert a couple of thin washers between the mounting faces, retighten and test the brakes.

If you go swapping boosters keep in mind that rod lengths vary between models and years for the same booster.

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LETMGROW · 10-04-2020, 02:28 PM

Not really being familiar with your chassis, you said you replaced the rear flex hose, are there any other flex hoses in the line leading from the master cylinder to the rear brakes? Check from the master cylinder down the line to the rear flex hose.
Lynn

TandW · 10-04-2020, 04:36 PM

I have just heard from a dealer that Workhorse brake booster units are no longer available. I have a 2001 Allegro 31IA that is rusting away up on blocks in SC.

RickNC · 10-04-2020, 04:46 PM

Quote:

Originally Posted by TandW

I have just heard from a dealer that Workhorse brake booster units are no longer available. I have a 2001 Allegro 31IA that is rusting away up on blocks in SC.

There are several companies that rebuild Hydraboost I had mine done for $150. The rebuild kits for DYI run in the $50 range.

daveDC · 10-04-2020, 06:13 PM

update !! first of all thank you everyone for the help. yesterday i replaced the brake booster and the master cylinder. i have only got to drive a very short distance (10 miles) but i think its fixed. the brakes were more responsive than they have been since i owned the motorhome (about 18 months)the rear drums were cool when i parked. tomorrow i will give it a longer test ride and will update with results. -

daveDC · 10-08-2020, 02:46 PM

UPDATE!!!!! not good news, so today i drove about 25 miles brakes were hot again!!! i am dumbfounded here. i can jack it up in the driveway start it pump brakes several times and the wheels spin freely. i am wondering if maybe the return line to the power steering is bad? i dont know. any input would be great.thanks

LETMGROW · 10-09-2020, 05:25 AM

It wouldn't have anything to do with the PS pump. If the return was plugged the pump would scream and you'd know it. You would be having steering issues as well.
By any chance have you checked the temperature at the differential or center section of the rear axle? Do you have an Infrared temperature tester you can check the temperature of the rear brake drums, backing plates and axle with?
When you jack the rear up and spin the wheels are the rear drums still hot or has the MH sat and cooled down?
Have you checked the fluid level in the differential?
This is risky unless you have a safe place to do it. You could use a tool made for the job to squeeze off the flex line to the rear brakes. Take the MH for a ride then check the rear drum temperature. This would tell you if it is truly a brake issue or rear bearings etc.
Earlier in this thread I outlined a procedure to apply and relieve the pressure on the system to help isolate the area where the problem is. Have you followed it or did you just pass over it?
Lynn