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

[jdrocci]

bsinmich

Thanks for that tip. How do you adjust the piston?

Joe

[bsinmich]

You could also be blocking the return of fluid if the MC is not adjusted to open the relief on the MC. Sometime the piston has to be adjusted for proper push length.

[jdrocci]

Upon further memory probing of what I saw while replacing the MC and looking at some pics on-line, I think this is a Hydraboost system, so it's a pressure boost, not a vacuum boost. Consequently, it's unlikely to be sucking and building up a pool of brake fluid in the boost system. I'm now leaning toward a combination of excess fluid level and boiling moisture in the system, causing expansion and pressurization of the fluid reservoir when the vehicle is running at low speeds where there's less air circulation to cool the system. I'm going to completely refill/bleed the front system and see what happens. It's the easiest 1st step.

Joe

[Go_Dawgs1]

Had the same problem with my 87 Allegro after I bought it. The previous owner had replaced everything on the brakes from the MC to shoes. The only thing he didn't replace were the hoses. 3 of my 4 wheels froze and had it towed to the shop and had hoses replaced. Never had a problem after that. When the mechanic took everything off he stated that he couldn't figure our why the hoses weren't replaced and if they had the problem wouldn't have occurred.

[Mainship]

Locking up can also be caused by the brake line rubber section at each wheel collapsing internally not allowing fluid to release and the bleed off solved it, for a short time.

[rgatijnet1]

From your description, the calipers hanging up may have been happening before you changed the MC. Mushy brakes and disappearing fluid could have been the result of dragging brakes and overheating the brake fluid and boiling the moisture out of the system. My main reason for going this way is because of the age of the chassis and the fact that unless the brakes are serviced regularly, the lubricant washes off and rust quickly form.

[jdrocci]

Thanks guys,

fordownr, Your suggestion to check for fluid in the booster is something I hadn't thought about. I had assumed anything sucked in would get sucked into the intake and burned. I guess I'll have to remove the MC to find out???

rgatijnet1, I'll try lubing the caliper surfaces also, but it's interesting this didn't happen until I replaced the MC.

[fordownr]

JD, if the booster if half full of fluid then it's possible. I keyed in on the Booster because of the MC fluid "disappearing" with no visable leakage. Heat is the common denominator. RG's post is worth considering also.

[rgatijnet1]

If the slides on your brake calipers are rusty, or not lubricated properly, they can keep the caliper from completely releasing the brakes. This can cause a rapid increase in brake temperature and cause the fluid to expand, especially if there is ANY moisture in the system. You will probably have to take the calipers loose and clean/lubricate the sliding surfaces to correct the problem.

[jdrocci]

fordownr

Yes, I thought of the booster, but why exactly now when the MC was replaced? And why only when hot at local road speeds? And why does releasing the pressure cure it for 100 miles of highway driving, only to return on local roads?

[fordownr]

Sounds like the booster.....(that may be where your fluid went)