1968 Mustang with power disc/drum and a tendency for the rear drums to lock up under more aggressive braking. Put an adjustable valve just before the rear brake line splits to each rear drum. Worked like a champ until two days ago. Brake pedal began to feel a little different, not quite as stiff. Pulled in to the garage, popped the cap of the MC and the front brake reservoir is flowing over and the rear brake reservoir is low. I transfer some the brake fluid to the rear reservoir and keep both levels about a 1/4" below the top. I check all four corners for leaks and find none. I drive it around town for a couple days and the same scenario happened, low rear reservoir and over full front brake reservoir. A friend suspects I blew an O-ring in the MC piston that is allowing the rear brake reservoir to flow in to the front brake reservoir. This seems to make sense since I have no leaks and when I transfer the fluid via syringe the levels are right back where they should be. Anyone have any good thoughts or suggestions?

 

Did you install this adjustable valve because the rear brakes were locking up? Or are they locking up now that you have a problem with the adjustable valve? After I installed a power booster on my car, which was previously converted to disc brakes, I had very early rear wheel lock up. I read that Ford had trouble with the big block Mustangs doing this when equipped with front disc brakes. Ford reduced the size of the rear wheel cylinder bores to balance the car's braking. I believe they went from just over a 1 inch bore to 7/8". I tried going from the stock wheel cylinders (for '66 all drums) to the smaller wheel cylinders for the factory disc brake cars, so from 7/8 to 11/16. Didn't help. I did this several times, reducing the size of the rear wheel cylinder bores until I got to the smallest ones available that still had the same mounting and bleeder/brake line orientation, which was a 19/32" bore wheel cylinder out of a 1978 Fairmont. I kid you not, they work great - and no "safe for off road use" adjustable valve.

 

Hey Mostang,

I installed the adjustable valve BECAUSE of the early wheel lock up and it works well. I was able to back off the amount of pressure to the rear drums and havent had an issue with early locking up since. But could that valve caused a failure in my MC? Its interesting to hear about Ford reducing the size of the wheel cylinders on the big block cars. Maybe I should look at those 19/32" bore wheel cylinders from the Fairmont and then I can get rid of the adjustable valve.

I'm still stumped about the fluid transfer from the rear brake reservoir to my front brake reservoir.

Brian

 

Mostang,

Did you mean a 29/32" for the Fairmont wheel cylinder? You typed 19/32" but which isnt as close to the Fairmonts listed .8125 wheel cylinder from 1978.

Brian

 

There is a path of failure that would allow fluid to flow from the back reservoir to the front, but not front to back. There is a third seal in there that keeps the fluid being pushed the aftmost piston from mixing with the fluid of the front reservoir. It appears to be called the "second circuit front seal". It sounds like yours has failed.

Yes, I used a 19/32" (.594) wheel cylinder from a 1978 Fairmont, 2.3 Liter, 10 inch brakes. I see they are a bargain on Rockauto for $.053. Assuming you have a 289/302 car, this would be about a 1/4" smaller bore. There are sizes in between. You have to make sure the bleeder and line fitting are in the same orientation as the original wheel cylinder.

 

I'd feel better if I could find the article on Ford reducing wheel cylinder sizes on big block Mustangs, but so far no. The concept is documented by other manufactures though:

Servo-Contact Brake Service (Session 187) from the Master Technician's Service Conference (imperialclub.info)

62-58.jpg (791×1024) (hamtramck-historical.com)