1976 F150 2WD
I discovered this right as I ran out of daylight today (no garage) and haven't had a chance to start looking into it. I figured I would get a little extra info from you guys in the mean time.

It appears at least 1 of my rear drum brakes is seized.

Possible Contributing Factors:

I just replaced the Master Cylinder. I followed the installation instructions exactly. I mounted it (after filling and bleeding it) loosely attached the brake lines and had someone push the pedal to the floor and hold it there. I tightened down the brake lines and had them release the brake pedal. It was nice and firm after that. I fired it up and it was still firm. I took it for a quick drive and the brakes were very firm with almost immediate response when touching them. Felt like driving a new 250 or 350 with aggressive brakes. It was nice. I didn't bleed the lines at the brakes at all since it wasn't spongy. That was all yesterday.

Today I got the horn working and took it for a quick spin around the block. I noticed immediately that the brakes were firmer than the day before. It got noticeably worse with each passing moment. It felt like the brakes were always engaged and when I would take my foot off the gas I could feel the truck decelerate faster than normal. By the time I got around the block and back home one or more was all but seized and it made backing into the driveway much harder than normal. I parked so that the truck was on a bit of a decline and put it in Neutral. It didn't want to move. I tried to give it a little push and could hear the back driver side drum rubbing.

The only other factor is that I need to replace a rear axle seal because it's leaking a little (1 reason it failed safety inspection).

So, if you have any thoughts or suggestions for things I should look into tomorrow, beyond finding out what brakes are acting up, I'm all ears.

 

If it appears that only one wheel is affected then that rules out a deffective new master cylinder. You'll need to pull the drum off and see what's going on inside.

 

I guess anything is possible, but I wouldn't think changing the master would cause the brakes to drag/lock up. I'd start by pulling the drums and do a visual inspection of the brake mechanism on each wheel. If all looks OK, consider exchanging the master for another.

 

Thanks guys. I wanted to make sure there wasn't some oddity with Master Cylinder swaps that I didn't know about and might cause this. I'll update tomorrow after I open her up.

 

So I took off the rear wheels and drum covers. Other than most of the components being a rusty pitted mess (but dry) everything seemed fine and the PO must have recently put new pads in. With the rear wheels and drum covers both off the brake pedal was still very firm. More reason to think the rears are fine.

So, I took of the front wheels. All of the pads were touching the rotors. They could spin but there was plenty of friction and noise as the pads rubbed. The rotors were a grooved mess. I disconnected the brake lines from the calipers with no noticeable change in the pressure on the pads.

I ordered new pads, rotors and calipers. I wasn't sure if the calipers needed to be replaced or not but I like the thought of updating the whole assembly at the same time.

With the brake line disconnected from the front left caliper the brake pedal was no longer rock solid (as expected).

Any thoughts on what caused the front pads to suddenly remained partially engaged all the time? To reiterate - no brake issues for the first 2 weeks since I bought it. Replaced the MC per instructions. Nice firm brakes, not spongy. The next day the front pads start refusing to fully disengage when I leave the house and by the time I drive around the block they are almost completely engaged.

 

It sure sounds like one or both front calipers were frozen... enough rust and crud got lodged in so there was no movement of the piston. You will know for sure when you put the new ones on. Why did it happen right then? Don't know. if the truck wasn't driven much before sale, built up debris and rust could have made its way to the calipers with the small amount of driving you did. Perhaps some crud got dislodged when you changed the master.

It wouldn't hurt to pump a pint or so of fluid through the lines before hooking up your new calipers.

 

With the rear brake drums off, the pedal should have gone way down and the brake failure light should have come on.
Have someone press on the brake pedal and see if the rear brake shoes are expanding out. If not the rear wheel cylinders are more than likely frozen and need to be either rebuilt or (prefferably) replaced.
Also, it's normal for the front pads to lightly drag on the totors.

 

The rear brake shoes did expand a little but it only took a little pressure on the pedal before it became rock solid. Now that I think of it I never fired her up and pressed the brake pedal while testing the rears, only the fronts. The rears were fully retracted and everything looked like it was seated properly.

Am I correct in thinking that if the backs were seized I wouldn't have been able to get the drum covers off so easily?

 

Not necessarily. If the truck has been sitting a long time and the rear wheel cylinders seize up from sitting, the shoes are in the retracted position so you would still be able to remove the drums.

 

According to the PO it was his daily driver.

 

I inspected the rear brakes and all brake lines with no apparent issues. I replaced the front rotors, pads and calipers and bled all the lines with no change to the "seized" brakes. I made sure the E-Brake wasn't at fault.

It turns out it was the Brake Booster Push Rod. When I installed the new Master Cylinder it didn't want to seat flush (like the instructions said it should). The Push Rod held the Master Cylinder mounting flange away from the Brake Booster by about 3/16"-1/4". I had also read that you shouldn't mess with the Push Rod adjustment and since I hadn't had brake pressure problems before I saw no reason to change it.

I believe this is what caused my problems. Because the Master Cylinder was always pushed in by that 3/16-1/4" it was never able to fully relieve all its pressure. This is why, after a little use, the pressure built up to where the brakes were always engaged.

I took off the Master Cylinder and adjusted the Push Rod. At first I backed the head of the Push Rod down too far and the result was very spongy and almost non-existent brake pressure. After I brought it back out, with 3 or 4 small adjustments, the brakes have the right amount of pressure and I drove it 15ish miles with no pressure build up.

For others who end up having the same issue: Adjust the Push Rod head so that, with it installed back in the Brake Booster, it stops the Master Cylinder from mounting flush by about 1/16". Then, with the Master Cylinder out of the way, thread the Push Rod head in another 1/4-1/2 turn so that the Master Cylinder is now able to mount flush. This should get you close to where you want to be. If the Push Rod head is threaded in too far you will have spongy brakes. If it is threaded out too far your brakes will continue to build constant pressure until it feels like they are always engaged (because they are). Once you get the pressure close to where you want to be make adjustments to the Push Rod head by 1/8 rotations. It might not seem like much of an adjustment but it is.

 

Thanks for the update, maybe you'll help others avoid the same situation. Reps sent.