power brakes on a 37 Ford problem

[topmoo]

I have a 37 that has a V8 (alas, a chevy 350) and power brakes, all done by the previous owner. I have done the interior and the exterior and am now getting around to some of the mechanical. The motor runs great and the truck drives great except! The brakes always grabbed when pressed even lightly, almost throwing any unsuspecting passenger through the windshield. I had a local shop "fix" them, they put a proportioning valve in and they did work slightly better. Disc in front and drum in back. Then the master cylinder went bad and to another shop it went. They have redone the entire master cylinder and booster three times and each time the pedal goes almost to the floor before it stopped the truck. The hurricane hit and the whole process got even more delayed. The brake shop ordered a new vacuum bleeder and the same thing happened again. When the motor is off the pedal seems normal, about half way it engages. When the motor is on it goes right to the floor. They can't figure out what is wrong and I suspect that the rod from the master cyl is the wrong length. They don't think so, others think that it is not bled right. Does anyone have any idea (assuming I have explained this well)?

[41inWV]

It's CONCEIVABLE that it's either the Plunger Rod is not long enough to engage the rear most bowl of a dual master cylinder. It sounds MORE likely to me that it's a bleeding problem. They CAN be a real bear to get bled right. Third possiblity is low manifold vacuum (assuming thats where you're drawing you booster vacuum from). On my '41, I added an electric vacuum pump JUST for the brakes, since I am using manifold vacumm for the C-4 transmission and the MSD Distributor advance. And the Flathead just doesn't produce enough vacuum for all three, so if my brake booster drops below 18 pounds the pump kicks on.

[topmoo]

Thanks,
That all makes sense. I will post what I finally find out.

[loholland]

topmoo, you didn't mention if your master is under the floor or mounted on the firewall, but if its under the floor MP Brakes says that you need a 2lb residual valve between the combination valve and the calibers and a 10lb residual valve between the combination valve and the rear axle. That way it holds the shoes near rhe drums and the front pads near rhe roters for a higher pedel. On a firewall mount master you only need the 10lb residual valve to thr rear drums.