1956 Stiff Brake Pedal

[CtCarGuy]

I've owned my 56 for about 5 years now and have always had a very hard brake pedal. The car has been converted to disk front brakes and the booster is connected to the front. I checked the pressure at the caliper and it is 900 to 1000 psi regardless is the engine is on or not so the booster does not appear to be working. I checked the vacuum and it is 20 in. I replaced the booster about a year ago with no improvement. I also replaced the vacuum check valve. Any ideas? I wonder if there was an error when it was installed. The hoses to the front brakes are new and the booster is connected to the smaller of the 2 sections of the master cylinder. Road test shows my back brakes doing the stopping. The disks look very smooth and shiny so I am going to have them turned and new pads. What I cannot explain is why there is no boost. I bleed the system including the booster. When I press on the brake pedal, there is no sound from the booster. Ideas?

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[biddle]

The subject is vital to getting more responses. When I saw "1956 Power Brake Booster" I thought you were selling or wanting to buy a brake booster I revised your subject to summarize your post which is about your stiff brakes.

 

[stevetheweave1]

Not familiar with 1956 model a picture of master cylinder might be helpful. Not sure of what you said " The car has been converted to disk front brakes and the booster is connected to the front." Since it is a custom install do they have a compensator/proportioning valve in the system?

 

[Critterpainer]

A proportioning valve issue was my first thought too. Also from your description of the Master Cylinder I wonder if your car has been upgraded to a dual bowl master cyl? If so, I have NO idea of how to plumb a hydraulic brake booster designed for a single brake system into a dual system.

 

[CtCarGuy]

Yes the master is a dual bowl unit. No proportioning valve or compensator. I looked at the instructions for one of the kits and intread of valves, they change the size of the rear brake cylinders. The real question is why is there no increase in pressure when the engine is running and the booster has vacuum.
I wonder if I got a bad one almost 2 years ago. I thought the brakes were better but that may have been just having new pads. I will be taking the rotors off tomorrow and checking to see if the pads are glazed. Even so, that would not account for the lack of any pressure boost. The booster only applies pressure to the front disks using the rear (larger) reservoir.

 

[74 Harley]

A picture would really help. I have been working on cars more than 40 years and cannot comprehend a booster only working on half a master cylinder.

 

[jimntempe]

A picture would really help. I have been working on cars more than 40 years and cannot comprehend a booster only working on half a master cylinder.

I have seen these conversions and it appears to me they plumb one of the dual master outlets directly to the rear brakes. So the rears have unboosted hydraulic pressure. The other dual master outlet goes to the inlet of the factory booster. The outlet of the factory booster goes to the calipers. So the calipers have boosted hydraulic pressure. Some of the kits balance the F-R pressure difference with a proportioning valve, others change the rear wheel cylinders to a different size to adjust the F-R balance.

A comment on the factory booster.... I rebuilt mine and I can tell when it "kicks in" if I apply the brakes while stopped... and it takes quite a bit of pedal effort before it hits the point where it starts to add boost. When rebuilding it I saw no obvious way to adjust where it "kicks in" and there was nothing in the instructions about it. It doesn't provide much boost. IMHO it's a crap booster design with multiple points of potential failure and many ways for it to be "right" while not doing much. I suspect it's only selling point was it was easy to add to a car with the manual brake setup without changing anything but two brake lines. Easy to add on the assembly line, easy to add at the dealership. WHen brand new maybe they even worked.

 

[Critterpainer]

Actually it is not a booster in the normal sense. Where I grew up they were referred to as Hydra-Vacs. Instead of the pedal activating the booster which then boosts the pressure being applied to the master cyl the master cyl pressure activates the Hydra Vac so it activates an auxiliary cylinder that does the boosting. They were quite common on 2 1/2 ton farm trucks in the 50's and 60's. The pedal feel is different and almost spongy as it is first applied with the engine running.