1987 F150, non-ABS, front disc, rear drums.

This all started with a bad MC I replaced it and bought a quart of Synthetic fluid (First mistake) and flushed the entirely original system components out with it. I have since found out that the synthetic fluid makes the old rubber hoses swell shut on the front discs and makes any seals that had DOT3 on them swell and fail to work properly, so I have changed: Both original calipers up front, Both Rubber hoses up front, the MC a second time, and flushed every bit of the synthetic out with 3 quarts of AutoZone DOT 3.

My problem originally started with just the bad MC that was worn out after 25+ years of hard service. After I replaced that and filled it with synthetic it ran good for about 2 weeks then the brakes all 4 started slowly locking up as I drove (took about 15 minutes of driving) and then would release after sitting for an hour or two. I tinkered with the rear drums and got them working right and havent had trouble with them again. The same issue witht he fronts continued for a few more days witht he brakes getting to the point of being locked by the time I got to work and then being fine by the time I am heading home. After investigating I found the right front caliper locked up and replaced both front calipers and hoses leading to the calipers and flushed the system with non synthetic fluid until I was sure it was good. Current problem is that now just the left (Drivers) caliper is locking down after about 20 minutes of driving and having it sit for long periods (even over night) it doesnt release. I can crack a bleeder valve and it releases. This led me to think that it was the hose leading to the caliper but when I took it off (while the brakes were locked up) and blew through it, it had no blockage. Any thoughts? Because I am definitely frustrated and dont want to stop driving the truck.

Paul

 

If opening the bleed screw releases the pressure then, either the hose has collapsed internally, the hard line is kinked/crushed somewhere or the master cyl is not letting fluid back in. You can check the master cyl by backing off the two nuts holding it to the booster about 1/8in and try the brakes then (will have excessive pedal travel but only a test). If no more lockup the very carefully shorten the push rod in the booster a little at a time.

 

Thanks I'll give it a shot before I head home today.

 

Thanks for the help! It seems to be working great now that I adjusted ~1/8" out of it. So far I have put 30ish miles on it with no brake issues.

 

Great. Then shorten the push rod a little at a time and bolt master cyl back up tight.