Mustangskip

I've been fighting a problem with the stock brake system on my 67 GTA (front disc, rear drum, power assist). I have replaced the master cylinder and brake booster, adjusted and bled multiple times, but the stopping power is just not there. Brake pressures are about right (1000+ front, 500+ rear), and the fronts seem to be doing their job, but the rear brakes just aren't engaging. With the rear end in the air, car in gear, the brakes will not stop the wheels from turning. Emergency brake will stop the wheels, so it has to be in the main system. I suspect that I am not getting residual pressure in the rear system, so it takes too much fluid to get them expanded out for a good stop. I read in one forum that the new MCs do not contain residual pressure valves, but I ordered the correct MC (power front disc, rear drum). I have tried a couple of different MCs as well. My question is, is the residual valve for a stock system supposed to be in the MC (rear circuit), or is it part of the Front Disc Brake Pressure Control valve p.n 2B091-B (for drum brakes) located back on the rear axle? Am I barking up the right tree?

2+2GT

The 67-69 proportioning valve can be rebuilt fairly easily, and for a lot less than $100.

You didn't mention the calipers- It's very easy to put them on the wrong side, and if you do, you won't be able to bleed them properly, they'll have air in them. Ask me how I know.

1965fastback

Nice post.

Mustangskip

Dan, thanks for the info. Some additional info and a couple of questions:

The brake shoes have not been replaced in some time, but there are very few miles on them. Visually, they are almost new with plenty of brake material left. The drums are original, but after turning, still within the maximum allowed diameter (according to the manual). I just replaced the wheel cylinders. When I adjusted the rear brakes, I adjusted them until they began to drag, and backed off very slightly (in fact, I let them drag just a little to make sure adjustment wasn’t the issue).

My questions: Can I visually (or some other way) verify that my MC (supposedly a rebuilt, stock, “correct one” according to how I ordered it) does have the RPV installed? Also, wrt to the “proportioning valve”, do you mean the pressure reduction valve back on the rear axle (Ford manual calls it a Front Disc Brake Pressure Control Valve)? If it was the problem, it seems I would be getting the same pressure readings between front and rear brake systems (or do these valves fail partially?). I have been considering a rebuild kit for this valve, but wanted to be confident this will fix my problem.

THUMPIN455

Take a look at the shoes, do you have two shoes with the short pad on the same side? Or do you have the short pads in the wrong spot front to rear? Off the top of my head I cant remember if the short one goes front or rear, but one shoe moves the other into the drum with leverage when the brakes are applied. It isnt only line pressure that does the stopping, they actually wedge themselves in there with mechanical leverage. The right shoe has to be on the correct side for it to work correctly. The parking brake doesnt use the same system it wedges both of them into the drum with mechanical means.

If that is ok, then start checking things like the proportioning valve and you did bench bleed the master cylinder before you connected everything right? If you didnt, then you probably still have air in the system,. Without a bench bleed it will take a LONG time to get it all out and your brakes will suck the entire time. The prop valve can stick to one side and make weird things happen, with a bit of time and the right seal kits, all the hard parts of the brake system on this vintage car is rebuildable. Its a simple matter of getting the parts back in the right oder after getting them out. Sometimes the getting them out part is the toughest.

2+2GT

Disc brake:


Drum brake:


Failure of the valve can result in equal pressure front and rear. This would mean the rear brakes lock before the front brakes do much at all. Air in the calipers would mean even the rear brakes would do very little.