P30 Brake Lockup - Solved

[jdrocci]

I have a 1988 Pace Arrow on a P30 chassis with 4-wheel disk brakes. Recently, the brakes have been getting pretty mushy and brake fluid has been disappearing regularly with no external signs of leaks. I refilled and bled the system several times, but the problem only got worse. Finally, last weekend, I installed a brand new (not rebuilt) master cylinder which I bench-bled first, filled with DOT4 fluid and bled all wheel cylinders.

When I took her out for a test drive, after just a few miles, the pedal started to rise until there was no free-travel at all and then locked up solid. I determined that both front wheels were hot and locked up. I opened the passenger-side bleeder valve, bled about an ounce of fluid out and the system immediately returned to normal. I chalked it up to overfilling the MC, which I know I did.

The next day, I took her on a 100 mile run and all was fine on the open road. However, when I got into slower-speed secondary roads, it locked up again. Another bleed-off of fluid and I got to my destination. On the way home, the same thing happened: it locked up when driving on secondary roads but was OK on the highway for 100 miles. As I approached my neighborhood, I could feel the pedal coming up though.

My first reaction was that it was due to overfilling the MC, causing a pressure lock when the hot fluid expanded, even though I never had this issue with the old MC. Some here have mentioned deteriorating flexible hoses, but I would expect a single hose failure to only lock up one side. Right now, I'm thinking of purging and refilling the fluid on the theory there might be moisture contamination causing excessive expansion when hot. I really don't want to believe it could be the brand new MC, and I can't rationalize how this could be caused by any MC failure I can conjure. I think a key clue is that it only happens after the engine is good and warm at local road speeds where there isn't as much air flow cooling the system.

Any suggestions appreciated

Joe

[The_Northern_Bu]

I have repaired this same issue on many vehicles. The front rubber brake lines swell and block off the brake fluid from returning to the master cylinder.

I realize from your last response this was not your issue but for those asking why are both lines going bad at the same time. It's because they deteriorate at the same rate. You will not even notice a brake pull. I've pulled these lines off and could not even blow air through them. What most customers notice is their brake pads wear out quickly?

Dave

[jdrocci]

Islander


Thanks for the input. For my education and for all the others promoting the hose theory, can you explain how a single defective hose can cause a problem on more than one wheel?

Thanks
Joe

[Ozlander]

Now that you adjusted the linkage, you can remove the washers.

Bad hoses can cause the brakes to lockup at all speeds, not just low speeds.

All mc's have a pressure relief hole, it called the fluid return hole, but it must be open when the brakes are off. Properly adjusted brakes can not build up pressure from overfilling the mc or boiling off water.

[jdrocci]

All

Today I got a chance to get underneath again and I think I solved the problem. I did 2 things, either of which or both cured it:

1) I loosened the bolts holding the MC to the booster and inserted flat washers between the 2 assemblies as shims.

2) Noticing that the brake pedal linkage seemed to be pre-loading the booster input rod a bit, I adjusted it a couple turns until there was noticeable slack,

I took it out for a long ride on local roads and there doesn't seem to be any more evidence of the pedal rising. The lockup is gone.

Thanks for all the suggestions

Joe

[jdrocci]

bsinmich

Yes, exactly.

[bsinmich]

That is another way of adjusting the push rod length on the MC.

[jdrocci]

okhmbldr,

Thanks for the interest. I've been trying to approach this in a somewhat scientific manner instead of just replacing things willy-nilly because someone said to try it. My approach is to develop a theory about what could be going on and perform a test to verify it.

My original theory was that an overfill of the MC fluid reservoir was causing a constant pressure charge on all the wheel cylinders when the fluid heated and had no place to expand. I tested for this by modifying the old MC cover to have vent/pressure relief holes with straws that extend down into the fluid. If the fluid was truly expanding in the MC reservoir, the pressure relief tubes would prevent a pressure build-up. I put this rig on a few days ago, but the brakes still lock up after just a few miles of local driving.

Reminding myself that this condition only appeared immediately after installing the new MC, I'm pursuing a new theory, that the plunger on the Hydroboost booster that actuates the MC piston doesn't have any clearance and is keeping the brake system constantly pressurized, causing brake drag and heating. This condition has been noted on the racing forums. I'm going to perform a test this weekend to investigate this. I plan to loosen the 2 bolts that hold the MC to the booster and insert some shims to keep them a little farther apart. If this cures it or improves things, I'll know I'm on the right track.

I'll keep you informed.

Joe

[okhmbldr]

I wonder if jdrocci got his brakes fixed? I'm curious to know if it was the MC or brake lines. I wonder if his unit has the proportioning valve attached to the brake lines? Just something else to check.
I hope he reports back a good solution.

[jdrocci]

Rustydog
Brake hoses are 5 years old or less. Also, both front brakes are locking, but not the rear, pointing to something in common with just both front brakes. I still have to ask how a single brake hose failure could lock up both front brakes.

Cajunfisherman
This isn't a vacuum boost system, it's a Hydraboost which uses power steering pressure.

This weekend, I'm going to drill a tiny pressure relief hole in the old MC cover and install it on the new MC to test my theory that the fluid is boiling, expanding and pressurizing the front fluid reservoir. If the scientific method doesn't yield any results, then I'll begin blindly replacing things like brake hoses, but I really don't like that approach.

[cajunfisherman]

I have had some brake problems in the past on trucks,all sounds logical but if calipers were stuck I don't think they would release with just bleeding the fluid. I would check that booster for fluid then check your vacuum pump lost my pedal on my 92 h/r with a ford engine and it was the vacuum pump went out. something to think about

[Rustydog]

If the hose were never replaced, replace them. You are assuming only one would fail at a time. All of your hoses are the same vintage.
My 94 coach gave me hose problems on an Alaska 2002 tour. I could not get the correct replacement while in Alaska due to a GM parts system error. They are cheap and not worth the trouble not replacing. Replace the rubber flex lines in the rear also.

[bsinmich]

I won't try to give an idea but I would definitely get a manual covering MC replacement and adjustment.

[jdrocci]

okhmbldr

Correct me if I'm wrong, but I'd think that would only lock up the one side where the hose is defective because the pressure is trapped between the blockage and the one caliper. In my case, both wheels lock up.

[okhmbldr]

Listen to the guys talking about the rubber section of the brake lines. I've had it happen. The brake fluid pushed though the line and forces the calipers against the rotors, but they collapse and won't allow the calipers to release. Locks up tight. You release the presure from the valve, and the calipers return, that's your clue.