Thread: Hot Rod braking problems

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New to this site and new to hot rods. Shade tree mechanic at best. Bought a car (1929 Ford Model A Fordor) already restored (retrofitted) with no knowledge of its components finished in 1981. But what I do know is the brakes are lousy. New master cylinder mounted on frame under driver floorboard. No vacuum or other power. Sits lower than calipers. Front calipers have one piston on Mustang II front end from 1978. Rear end is from 1979 Corvette and has four pistons. I do not know the size of the bore on the master cylinder or either caliper. It has an adjustable proportional valve plumbed to the rear brakes and Wildwood blue 2 psi residual valves plumbed front and rear. Excellent pads and rotors.

The brake pedal is full and hard, not spongy but you have to press down hard to make it stop and it will coast to a stop with no skidding or throwing you thru the windshield.

Does anybody see any problems with this setup that may help me short of disassembling the master cylinder and/or calipers and provide that information? Are the residual valves the right psi given the multiple caliper piston arrangement and would they be the problem? Are they needed?

The pedal comes out of the floor, makes a 90 degree turn to the right (if you are sitting in it) then another 90 degree turn towards the driver. It does not appear to be binding in any way.

Welcome to CHR!

Might sound silly, but have you flushed and bled the lines? Sometimes crud grows in brake lines - especially if she been sitting for a while. Might be worth a shot.

Regards, Glenn

Welcome to the site. Maybe you could share a pic of the car with us?

Have you even looked into the master cylinder? Like Glenn above, I'd be curious as to the color / condition of the fluid.

I am also curious as to the brake pedal length. I wonder if the builder change the mechanical advantage during fabrication by making the bends to place the pedal where he wanted it.

Could you measure the brake pedal? We's need to know the 2 distances from the pivot. Pivot bolt center to the connection to the master cylinder center and then pivot center to the pedal itself center.. The mechanical advantage is the "horsepower" you can input to the brakes hydraulic system.

My other thought would be to lift the car completely off the ground and have a helper apply full pressure to the brake pedal while you try and turn each wheel. I'd be curious if any of them turn. Even if you used a long prybar ( like a diggers rock bar) off the floor and really work at making a wheel turn.

Well, that's all I've got for now. We'll see what others come up with.

Welcome, I'm also wondering if you don't have enough leverage on the pedal.

Provided the brakes are in good mechanical shape, it sounds like low line pressure. Could be too large of a master cylinder bore or not enough leverage in the pedal or both.

If I recall right, the original MII master was 7/8" bore. You need to shoot for at least 1000 psi with a leg effort around 100 lbs. Disc brakes require a lot of pressure.

Here's a couple of online calculators that will help.

https://www.markwilliams.com/calculators.html

https://techtalk.mpbrakes.com/how-to...ng-pedal-ratio

You could also have a mechanical issue. Stuck caliper pistons, stuck floating caliper (would only apply to the fronts), badly glazed rotors/pads, wrong pad material would be few possibilities.

Edit: I was close on the MII MC bore. It's actually .937". 7/8 is .875.

Good call on the fluid - is it a nasty color or nice and clear?

How much travel does the rod going into the master cylinder have? Generally speaking a pedal ratio of about 6:1 should be adequate, depending on the size of the master cylinder. One other thing to check is if the residual valves are installed correctly, there is an in and an out side to them. Was the master cylinder bench bled before it was installed. I don't mean to be insulting with dumb questions, but often the easiest things are overlooked and can cause major problems.

I have a 1936 coupe with the same basic setup. 9.25" rotors in front and a Ford 8" in the rear. Mine stops as hard as my drum brake 57 Chevy. I dont think the small rotors cut it for stopping power. I plan on going to a 11" Wilwood setup. Its four piston and low profile so I wont have to go to a 15" wheel.