Hi guys I made a post a couple of months ago about the brakes on my 1979 f250 acting funny. It seems like that first half of the pedal travel has little to no affect on anything then very suddenly the brakes start applying. It’s unsafe to drive. I’ve been putting it around a couple of easy back roads and it’s safe enough for that just to test them out. It made it next to impossible to apply brake pressure without coming close to locking the tires.

I put a new master cylinder on it and after bleeding the brakes really good twice it’s slightly better but still doing it.

I tested them today on some slick dirt at around 15 mph. The rears locked up, and then with the rears locked and the fronts not the truck continued to roll/slide forward pretty easily. Almost like the fronts weren’t really braking but the backs had already locked up. This is at about 75% pedal travel. Closer to the floor than the top. Not something I would thing would have thought would lock the tires up, but still enough pedal I feel like it should have been stopping faster.

I’ve done a ton of work in the 3-4 months on it and the brakes just keep me from being able to drive it.


They went “bad” after the truck sat for 2-3 months. Before that they felt great and worked great. I didn’t do anything to the brakes except remove the brake booster to do some other work behind the dash. I suspended the MC from the hood instead of unbolting it while I was doing that.


Anyone have any ideas? I’m gonna go through the front brake lines soon and make sure none of them are kinked but I’ve done that already.

 

Almost reads like the actuating rod that lies between the booster and master cylinder is adjusted out too far.

 

I really need to figure out something. The truck has had great brakes for 10 years now.

I didn’t adjust the booster rod, the nut is actually rusted in place. I’m going to adjust it back and forth a little to see if that makes any difference. I’ve already triple checked my booster and MC mounting.

 

Have you tried tightening up the rear brakes with the adjusters? Or checked if they need it

 

I'd check the rear brakes in general... i've had the hub seal leak into the rear drums and it feel like you're describing as well as beardedcap's suggestion of the auto adjusters sticking and not adjusting out like they are supposed to on their own.

 

I would bleed the master cylinder again. Check proportioning valve.

 

I’ll jack the rear end up and make sure the rear brakes are as tight as they should be. The parking brakes works the same as before.

Im going to check all the lines from the MC to the proportioning valve to make sure I didn’t rough house one of those when the MC wasnt bolted on.

I’ll get under there and rebleed it too. I bled them multiple times before changing the MC. After changing the MC they bled better but still malfunction like I’ve said.

 

Can you confirm that the front brakes are working? Jack it up and have someone spin the tire when you apply the brakes. If both aren't working, I'd suspect master cylinder issues. I'd also think about flushing the brake fluid and changing the flexible rubber hoses if they haven't been changed for a while. Sometimes an old flexible hose will swell up when you apply the brakes and that adversely affect your braking.

 

The rears on these trucks will lock up first doing the dirt/gravel road test with no load in the bed. But if you brake sudden and hard like a panic brake all 4 should lock up.
Can you pump up the pedal at all with 2 or 3 pumps? Pedal travel can be subjective but 75% down seems like too much.
With discs in front, too much pedal travel is often a product of poorly adjusted rear brake shoes, incorrect installation, or mismatched parts. As soon as the brake pedal is moved, the master cylinder is moving fluid. That fluid has to go somewhere. Too much wheel cylinder piston movement before the shoes contact the drums is an example.
The other possibility is the master cylinder is not actually pushing fluid and instead leaking past the piston seals internally. New master cylinders can fail.
What method are you using to adjust your rear brakes?
If you want to verify your pushrod is correct. Loosen the bolts maybe a 1/4" or so on the master cylinder. Push brake pedal and see if the MC pushes out with slightest movement of pedal. It should.
Old brakes hoses can collapse internally but most often the symptom is slow or no return of caliper of wheel cylinder pistons causing drag/heat.