Hi guys,
I posted a turn signal question yesterday and am very impressed with all the advice so far, its really nice to get advice, ya know. Well anyway I have another question regarding my 78 F150 4x4. I am having a problem with the brakes, it seems like the rears are locking up earlier than the front ones. Also the weirdest problem is that brake fluid is being forced out of the top of the master cylinder. Almost like it is building excessive pressure due to a blockage somewhere. I am suspecting that it might be a bad proportioning valve. I have replaced the master cylinder twice as well as the booster and keep having the same problem. The pedal seems to not do a lot without really forcing it to the floor. I haven't found any other leaks anywhere and I have bled the brakes several times and they are all getting fluid. Any advice?

thanks

 

I agree with your diagnosis, mine went bad on my 77 4x and i was able to take it off and pull it apart while using compressed air to clear my blockage, It worked and im glad because they're very hard to find aftermarket

good luck

 

Wouldn't an internal leak in the master cylinder give the same symptoms?

 

The plunger in the MC is not travelling far enough. It may be returning too far as well.
The explanation is long, but look into that problem.
Also, on my 73, I replaced the MC and it turned out I had a late 72 so the MC for a 73 was the wrong one.
You might look into that. Maybe your MC was replaced before you had the truck?

 

I would like to know more about this plunger thing, I know that the MC is the right one, as I have replaced it two times, but I'm thinking that it could be the proportioning valve.

 

i redid my brakes on my 77 completly. my proportion valve was bad, had to go to the dealer and it set me back 200 bucks, try taking the valve out, and pull it apart. there is a piston inside that maybe if you clean yours it may work, also, there is a way to reset the valve after you put it back in so that the piston is set correctly. I had to look at a chiltons book and that helped ALOT. i also did my brakes like a yr 1/2 ago so i may have forgotten something, but try cleaing it out and reseting it.

 

I have to side on the Proportioning valve is bad.

A leak-by in the MC won't cause Brake lockup like a BPV will.

The leak-by is caused by a bad seal on the piston for one of the systems.

 

The Rear's before the fronts are locking up?
That should tell you the Proportioning valve is not working. That's the way the brakes worked before the valve, and the reason the valve is there!

You said the fluid is being forced out the top of the MC. (with the top on or off?)
If the fluid is spurting out when you press the pedal down, it means the pressure is building before the plunger seal has passed the Hole in the chamber of the MC.
Something is locked up down below!

If the fluid is spurting out when you let the pedal off, that's from too much pressure from the Shoes/Drums back up to the MC. Although it always spurts when you let off the pedal and the fluid returns to the Chamber in the MC.
Sometimes, especially if you have air in the lines, or the Drums are weak (too thin) that apurt will shoot up and paint the hood for you. (with the lid off of course)

So you have swapped the MC twice? That still does not mean you ever had the correct MC to begin with.
You may have replaced the MC with a look-alike, and put a rebuilt-but-wrong MC back in there.

From reading all these guy's post's on Brakes, I know I've lived a charmed life, but in all the Ford's I've owned, I've never had a problem with the proportioning valve.

One more tip, this is a good thing to do, but it's a tip born of Desperation.
This stopped a problem on my 77 and on an 81 bronco. Both suffered from rusty drums.
I bought two quarts of Brake fluid and a Vacuam pump.
I pulled all the fluid out of the system, blew out the lines, and replaced the whole dirty messy icky works with new fluid.
I pulled the new fluid through with the vacuam pump, rear's first, and then the fronts.

I replaced the Wheel cylinders n' Shoes n' turned the drums in the rear.
On the front I just put in new pads. Then the new Fluid and a long time bleeding, and the truck had tippy toe brakes again.

 

Man you guys are awesome, this is the only place where I have found anyone who knows what they are talking about. I've even had it into the shop and they just say the mc and booster are shot. Well I guess I now have a good place to start, but I'll have to wait until a warmer day, its only like 5 degrees out right now.

 

Your booster may be shot! I've had that too. The only safe way to handle that is with a new one.
Sometimes a wrecking yard booster will work, but not often enough for me. I want good brakes.

 

The rear brakes are designed to be applies first followed by more vigorous Front brakes doing most of the braking.
The fact that the rears are Locking up is bad.

I always like to evacuate ALL the old brake fluid when I am doing brake service work.

Brake fluid absorbs water & that's bad, plus, the brake grime needs removing.

that's from the rubber parts breakdown from use.