model a hydraulic brake problems?

I have been researching on here for some answers, got a few problems fixed but still have brake issues.
First I'll list what I have set up. 1931 ford model a 5 window coupe, 67 corvette master cylinder from napa (new not rebuilt) made for manual drum/drum.
1946 ford deluxe backing plates and drums. new shoes all around, wheel cylinders in front are new back are good. new brake lines all around, new rubber hoses in front and new one in rear.
I have a cross shaft from the driver side to the p***enger side because the master cylinder is under the p*** floorboards.
My problem is that I have next to no brake pressure stopping the wheels. I have adjusted all 4 wheels as far as the shoe adjustment goes. I have an emergency brake that does work when pulled.
If I disconnect the push rod that goes in the master cylinder I get more than an inch of travel so I know there is no pedal binding or limitations as far as the pedal goes.
Now when I hook it all back up I get a stiff pedal and no brake pressure. If I have a friend push the brake pedal I can feel resistance on the wheel but I am still able to turn the tire with a little force. I dont get as much brake pedal travel with the push rod to the master cylinder hooked up either.
I have no residual valves, 10 or 2psi pressure things hooked up or any proportioning blocks. pretty much straight brake lines and tee's . The front (slightly bigger reservoir) is hooked up to the front and back(small reservoir hook up to the rear.
I think I covered as much as possible.
thanks josh

 

Have you checked the overall pedal ratio, including any change of ratio in the cross shaft?

 

I got that but with the 46 master. It works great. I would play around with the ratio. You need more leverage. I had a problem and i think i had to add to the arm off the cross shaft.

 

Measure the distance between the pivot and the pedal. Measure the distance between the pivot and the push rod attachment point. That is your pedal ratio. Post your results.

 

if you added to the arm off the cross shaft you decreased the pedal ratio meaning that it take more force to stop the car. You also need to know the bore of the Corvette master cylinder if it is larger than 1" it will also effect your brake performance.

 

as i read it this setup worked fine with a `46 master cylinder but not with a corvette?

the `46 master cylinder has a built in residual valve , the corvette does not

 

Not a 67... 63-64 yes but not a 67.

 

I had the 46 single reservoir in the car and it was also the same weak brakes. I do feel the new corvette m/c is a little weaker. The bore is 1 inch on the corvette mc. Where should I change to improve pedal ratio? Both arms on the cross shaft are about the same length. Maybe the arm going into the m/c on p***enger side might be a slight bit longer

 

Leverage in increased by either making the brake pedal arm longer (may not be possible: ergonomics/practicality), or making the arm that pushes the pushrod shorter.

Keep in mind, the shorter that the pushrod arm is, the more pedal travel you will have.

 

Are you sure that you have the brakes adjusted properly???? sometimes those flathead backing plates can be confusing...

 

As already noted, it appears like your pedal ratio is the problem, or major mechanical losses through the cross shaft setup. A picture or two may be of help, but you want a 6-7:1 overall ratio.

Also noted previously noted, your master cylinder is in question., but probably isn't the problem. Corvettes had single system 4 wheel drums up to '64, then single system 4 wheel discs in '65, and finally dual system disc in '66/'67- they never had a drum/drum system.

You have the master plumbed correctly with the larger reservoir going to the fronts, but you need residual valves with your vintage drum brakes, either internally in the master outlet(s) or external 10 lb. valves.

 

I have a similar setup and it wouldn't work with out the residual valves.

 

The ford cylinder you took off has a 1-1/16" bore. The corvette (I'm guessing) is a 1" - could be a 1-1/8 - you would need to say which.

If it is a 1" bore you have lost some area. I'm guessing you are going full travel and not moving enough fluid to prime the system and get some pressure built up. If the ford cylinder was a bit sketchy before the swap to the corvette cylinder it was probably due to the same thing - insufficient travel.

It may be worth fitting a 1-1/8" cylinder - this may swing the balance just enough so that you move enough fluid to get the system filled and then allow pressure to be applied properly.

With this linkage you have, there may be some sort of flex or lost motion. if there are levers on shafts, make sure nothing is moving - your safety depends on it.

Good luck with it.

Mart.

 

I've just gone through this with a Mustang m/c. Turns out that because the m/c is under floor now, where it was originally firewall mounted, that it needed the 10lb residuals to stop the brake shoe springs pushing all the fluid back out of the wheel cylinders. In a firewall mount this is achieved by gravity. Apparently.

 

A 10 lb. residual can in no way over come the powerfull shoe return springs, and prevent the shoes from fully returning to their anchors. I know this is believed and spouted by many, including some leading brake componet suppliers, but it's plain BS. The valves will pervent air from entering past the wheel cylinder cups, as they were designed to do in vintage drum systems.

 

Because you have a hard pedal but no brakes, this indicates to me the displacement of your master cylinder is too small or that the master cylinder push rod is not adjusted correctly and you are braking in the front or rear but not both. Jack the vehicle up on 4 jack stands. Have someone push down on the brakes. Question, If you step on the brakes can you turn the front by hand, can you turn the rear by hand. If one end locks up then you have the pushrod adjusted wrong. If none lock up then you have a mastercylinder that is too small. If you were able to stop with the stock Ford cylinder but not well it means you have a lousy pedal ratio.

 

I thought the 67 was a drum/drum. Should I get a new m/c maybe from something like a 1969 ford bronco that was a drum drum mc. I know because I had one?

 

I did jack up all wheels at the same time. The front were starting to grab and the rear was also starting to grab but spun a lot easier.

 

Thankyou for correcting my wrong ***umption.

 

We used to use 67 Mustang M/cyls for a dual drum/drum brake system. Will others work?... probably. you need to ask for drum/drum since disks were optional. It always worked for us so I was not eager to change.

 

Ok thanks. I will probably first change the mc to a 67 drum drum setup. Then work on the pedal ratio. Hopefully that solves my issues.

 

I'm looking online at the pics of the 67 mustang mc Would the drum drum mc have almost same size front and back reservoirs or is the one significantly larger than the other? It's lists the one with the same size reservoirs as manual brakes. The ones listed as power brakes has the larger reservoir in the back towards the firewall. Sorry for the questions, don't want to get the wrong one again

 

i think your master is fine, look at the pedal ratio first, the size of the reservoir has to do with making sure there is enough fluid so that when the shoes or pads are worn out you still have fluid left, your one inch bore master should work fine.

"If I disconnect the push rod that goes in the master cylinder I get more than an inch of travel so I know there is no pedal binding or limitations as far as the pedal goes"

so can you measure how far the pedal moves when you get your one inch of rod travel?

 

From what I could tell doing it by myself I have 2 inches of pedal travel and the end of the shaft for the mc moves one inch

 

i would say thats your problem, you should move the pedal 6" and have the rod move 1", that would be 6 to 1 and that should be your goal.

 

Yikes! No wonder you have a hard pedal.

 

The mastercylinder is lower than the wheel cylinders when its mounted in the floor ... Gravity makes the wheel cylinders blead back down to the master cylinder causing air bubbles in brake fluid ... Thers a valve you can get to put in the brake lines to fix the problem ... Saw that on chop cut rebuild last night ...

 

Harry, that is the residual pressure valve. Not having one will not make for a rock hard pedal, but could make for a spongy one.

 

Ok. Now I have to figure out where to change the linkage or leverage arms. I know the pedal is pretty straight forward and can't change anything there. So it's either going to be the arm on the driver side or the p***enger side of the cross shaft.

 

So, if you have a linkage between the pedal, to an arm on a cross-shaft, drill a new hole closer to the shaft on that arm, and attach the linkage there. I would start on the arm that is not the one on the master cylinder, and see if that does the trick. My rationale behind that is that you want the pushrod pushing the piston in the master cylinder as straight as possible. Relocating the pushrod hole will change the push angle. Pushing at too much of an angle can accelerate wear in the cylinder bore and/or the piston. With an aluminum cylinder, it can be a quick demise.