Hi all, I have a 1981 Ford F100 Custom 2WD. Straight 6. I've replaced my master cylinder recently, and now my brakes are starting to lock up. I bench bled my master cylinder before install, and I bled my lines with a cheap bleeder gun from Auto Zone in the recommended order, furthest from the mc, first etc.. I did notice that the brake fluid was just barely flowing while bleeding. The brakes work for a few miles, then they lock up. After about a day, they're fine again and the whole process starts over. What did I do wrong? Any advice would be greatly appreciated. Thanks for reading.

 

Is it the front brakes or rear brakes locking? Both wheels or just one wheel?

Could be a sticking caliper or piston, also rear brakes out of adjustment can cause one wheel to lockup faster than the other during hard braking. Problems due to heat from braking is more common in front brakes where it causes piston to stick, then it cools down and works okay for a while. Also, sometimes dirt/debris in the brake system around the caliper piston will cause a problem such as when the pads are replaced as the pistons are pushed back in their bore where the dirt has accumulated. Perhaps you stirred up the debris in the brake system/lines and now its causing the piston to stick in the caliper.

More details please.

 

The spec below is very important. If the master is being pushed ever so slightly, it will cover up the compensation ports in the bottom of the master and cause the problem you describe.

The push rod should extend past the face of the booster as follows:



1976: 0.880-0.895 in.


1977-81: 0.931-0.946 in.


1982-86: 0.980-0.995 in.

 

CountryBumkin: I think it's the both in the front, that's where the "funny" smells and sounds were coming from after a quick trip down the road a few miles to pickup dinner for the family. I did nothing else but replace the master cylinder so I didn't think rear brake adjustment would be an issue; could I be wrong in that assumption? Thanks for the advice.

 

Are you freaking kidding me?! Wow, did not know that, thanks for sharing.

 

UPDATE: I went home and messed with truck and noticed that once again the front brakes were locking up after a short drive. When I got home and put Ol Blue in reverse, the idle was enough to move him backwards. In drive however the brakes were keeping him from moving until i gave em some gas. Anyone have any ideas?

 

I don't think the moving in reverse but not forward means anything. The gearing in your tranny may be lower in reverse than 1st providing a torque advantage in reverse.

Try this, drive a short distance (5 miles) then immediately jack up the front end. See if the front when spin freely. Try again when cool. Obviously they should be free whether hot or cold.

Also, note that if the rear brakes are not working properly (sticking, out of adjustment, stuck parking brake cable, or faulty proportioning valve) it could make the front brakes seem like they are malfunctioning because you have to apply more brake pedal effort to stop the truck.

I had a similar issue once, where after driving a while the front brakes would drag (actually smoking). It turned out the my calipers (pistons) were sticking. A new set of rebuilt brake calipers fixed the problem.

 

Along the same theme as my 1st post in this thread, I have a little ranger pickup that I got for free, it had the same problem and had been passed from one person to another, lots of parts replaced, and still the brakes would lock up after it was driven a few miles.

It turned out to be the booster. The booster was rusted and sticking, if you put your foot under the pedal and pulled up on it, it would not do it. Lots of wd40 didn't fix it either, I had to buy a rebuilt booster.

But the cause and the affect were the same. The booster was not releasing totally, slightly pushing on the master, which covered the compensation ports, and when the fluid gets warm it expands and instead of being able to expand into the master cylinder, it starts applying the brakes. Just because that little hole in the bottom of the master is blocked off.

 

So, did you adjust the pushrod? Classic symptoms, do it NOW lol ~Bill

 

Thanks so much for the advice. I will give it a shot. I'm thinking of going ahead and replacing lines and calipers just to be on the safe side. I mean, it certainly wouldn't hurt and seems as though it's fairly reasonable cost wise to do in a weekend. As long as the parts are available.

 

I haven't though about that either. I will definitely look into that as a symptom as well. Thanks for the input.

 

You know what, I didn't adjust it. I didn't have much light to work in when I got home last night that I was trying to see if only 1 tire or both were locked up. This is definitely going to have to be a Sunday morning project. Thanks for the reminder!

 

Maybe your new master cylinder is bad? Have you tried reinstalling the old one to see if you have the same symptoms?

 

I thought about that as well but I core exchanged the old master cylinder so I no longer have it. I think I'm just going to have to get start from scratch tomorrow and do a few things at a time. What will probably happen is replacement of the whole brake system. I don't want to be driving my kids around and inadvertently put them in danger. Thanks for the idea, I appreciate it.

 

This post reminded me that I had the same problem as Dave describes, once too. The easy test is get your foot under the brake pedal and lift. If the pedal comes up and the brakes release you know you have a sticking booster. However, in my case, the pedal would stick after each brake application and not just when the brakes/fluid was hot.