P30 Brake Lockup - Solved
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
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
