Hi folks,

My newbie post here, I now it's long but I want to give plenty of info. I just acquired a 1978 F-150 custom w/ 350M / auto / 2wd. My problem is that when driving the truck the front brakes will lock up. It seems to happen when I hit a bump, not necessarily when I apply the brakes, and once it starts it grows worse as I drive. It only affects the front (disc) brakes.

The problem began as soft brakes when I bought the truck. I bled the brakes but the front callipers would not bleed properly (could never get all the air out). I installed a new master cylinder and the front callipers bled properly. At the same time I replaced badly worn hoses from the hard lines to both front callipers. Test drove without incident but the next time out the fronts locked up. In the 30 minutes it took me to go home and grab tools, the brakes had mostly released on their own.

Since I had tools anyway I opened a calliper bleeder valve to relieve the remaining pressure and of course they released. I don't believe both callipers would fail at the same time, so I suspected the new master cylinder or a bad proportioning valve. I drove until they locked again, then opened the front brake line at the master cylinder and it released pressure there, confirming that the master cylinder and not the pro valve was holding pressure on the brakes.

So now I'm thinking it must be a bad master, but.... is there any way the vacuum booster (yes, power brakes) could be applying pressure to the cylinder when the truck bounces? It makes no sense to me... the second time they locked I wasn't even pressing the brake pedal! This is fairly "normal" behavior for my drive-by-wire Jeep but I don't expect ghosts in these old mechanical systems!

To be clear on what I mean by locked up.... the brakes apply (front only) causing the front end to bounce. The wheels do not actually lock up and squeal down the road, the brakes aren't that strong, but if you stop then try to go the rear tires break loose and spin before you overcome the brakes. They remain "applied" until the truck sits for a while and after 20 minutes or so the front wheels will spin again. As a note I did not find any leaks, it seems they return to normal as fluid returns to the master cylinder reservoir.

Any ideas out there? Is this just a haunted master cylinder or could there be a vacuum problem going on?

 

New master cyl, or reman? Sounds like it's just a defective unit. Could have a plugged port internally, or simply the wrong internals. Wouldn't be the first time I've seen an improperly built brake master.

 

you did bench bleed the master cylinder before you installed it, right?

 

It had to be the master cylinder
Check out the wierdest problems thread in the 48-56 forum some Guy had a rod in the master cylinder slightly too long

 

X2 on the bench bleed.
If you did not do this you may not get all the air out of the system.

 

Yes, reman.

Yes, bench bled the master. Interesting point, the rearward reservoir (front brakes) had to be gravity bled, pumping the piston would not bleed it, but it did push fluid once it gravity bled. This was the third rebuilt mc, the first two would not bleed at all. Guess it's just a bad run of master cylinders.

Thanks for the input, this is the first time I've replaced a brake master so I needed confirmation to make sure I wasn't overlooking something.

 

I'll bet $10 you haven't been pulling the pin on the proportioning valve when attempting to bleed the fronts...

 

Does the new master/booster have an adjustable con rod going to the pedal? If so try backing off the connecting rod adjustment a turn or two, and see if that improves the problem! It might be slightly depressing the cylinders in the master.... also some of the older trucks that have Disc/Drum set up, and little inline check valves to maintain the proper pressure separate of the Proportioning valve..... as a rule the fronts have a 2psi back flow check and the rears would be 10psi valve... they might be plugging up! Just a few thoughts!

 

I have been holding the pin out while bleeding the fronts.

I haven't pulled the booster yet, so I'm not sure if there's an adjustment there, but the end of the rod that hits the master looks to have a tip that threads in and out making it adjustable. I'll take another look... I think you're on to something there! I was not aware of any additional check valves... however my "test" for the pro valve of locking them up then checking for back pressure at the master should resolve the question of any major backflow issues downstream.

 

Did you get the brake rod adjusted? Here's how to check/adj it:

Click on pic to enlarge and again to get detail.



.

 

Interesting... the adjustment screw is pretty well frozen, but even if I could adjust it back it would never get to ~.995" as indicated in the diagram. It's ~1.325" out now. Perhaps this booster is not the original to the truck, so while I bought the right master for the truck it may not be the right match for this booster. I've already returned the original master core so identifying this thing might be difficult, I'll see what I can find.

 

Measure the distance to the bottom to your existing masters bore and compare that. All you need is clearance so there is free play when the brake is at rest. And start soaking the adjuster with penetrating oil now....

.

 

No luck... even if I run the screw all the way back it will still depress the the master by about 1/4". I could space the master off the booster with washers or nuts to get the right length, but that seems pretty weak! I googled the numbers stamped in the booster but only came up with a cardone booster for a 72-73 dodge truck. Probably a fluke, I can't imagine this is a booster off a dodge. I'm thinking a new booster, this one isn't doing much anyway. Just wish I'd known when I started, the new booster + master is about the same price as a booster by itself!

 

Does the pedal have free-play? Could it be pushing the rod further out than it should? Can you post a pic of the booster? I think a reman booster/master combo from O'kragens was about 80 bucks but make sure the line nuts are correct for what you have now.

.

 

No play in the pedal, it has a good spring return. The rod is not attached to any linkage, it seats back in this position but can be pulled forward and will flop around in the booster. I'm measuring with the rod pushed back as far as it will seat. I'm a newb here so not familiar with pic posting limits, I'll try 650x400 and see if it will take, that should give enough detail w/o being HUGE....