Power Brake Problem - Could use some help

I am working on a friends 1956 Chevy (recent build) that has Disc Front / Drum Rear brakes. It has a 7" double stage booster with a factory style proportioning valve. Looks exactally like this one:
http://www.speedwaymotors.com/1955-57-Chevy-Brake-Booster-Combo,1984.html

The problem is there is not suffient braking force. Even when I stand on the brake hard it will not lock up the wheels. normal stops take a lot of effort and the car rolls way past where I would consider good stopping distance.

The original builder said it has been this way from day one.

Things I have done so far:
Bleed the brakes. Verified no leaks.

Run tests on the Brake Booster:
(1) Check Valve - Checks OK
(2) With engine off pump brakes to remove residual vacuum. Hold pedal and start engine. According to the trouble shooting guide the pedal should drop about 1/4". This car may go a little farther like 1/2"
(3) Run the engine then turn off. pump the brakes slowly. 1st pump should be low then the 2nd and 3rd slightly firmer. - Checked OK
(4) Start engine, hold brakes, then stop engine. Pedal should remain firm for 30 seconds or more. - Checks OK

Vacuum is about 18 inches

So, it appears that the brake booster is functioning based on the tests.

I seperated the booster from the master and adjusted the push pin to the ideal length. Verified that the pin is extending and retracting when the brake pedal is pushed and released.

Also tried both holes in the brake pedal arm with no change in braking force.

So I guess I am down to the master cylinder and proportioning valve. Unless I am missing something abour the booster.

Any advice would be greatly appreciated

 

You don't have front and back brake lines reversed at the master cylinder by any chance?

 

use vacuum gauage . and advance engine to about 21 pounds and sell if this helps those 8 inch doulble boosters may need more

 

Master Power Brakes has some good tech. There is a page with pics of various master cylinders to show which port feeds front and rear brakes

 

Yes I did verify that the front and rear lines are connected to the correct ports on both the master & porportioning valve.

 

maybe check the lines for blockage? remove all the fluid and spray some brake clean through the steel lines till its clean (i wouldnt do this on the rubber lines because ive know brake clean to eat rubber). also seeing as how it has had a lot of recent work, check the size of the lines, if they are too large they wont be moving enough fluid to engage the pads/shoes and if they are too small they wont move enough force.

 

possible the master has the wrong bore size. those kits usually use something like a mustang master cylinder which has about a 1" bore, which should be plenty. also note that we see alot of master cylinder failures with those kits. they're cheapo china units most of the time. take the master off and see if there's any wet at the bore. also try bench bleeding the unit just for giggles.

you may have some collapsed brake hoses or stuck cylinders/calipers

if the pedal feels hard as you're standing on it, the booster may be bad.

the booster may just not be big enough. we had that problem on a 65 impala.

 

Sounds like air trapped in the system. Are your front caliper bleed screws on the top, seen them reversed (me being one) giving that symptom. Also read that air would get trapped in the thru the frame fitting of some of those types of fittings.

 

Caliper bleeders are at the top. Brakes are not spongy.

 

Check front caliper piston for expansion and release operation. Check rear brake shoe adjustment and operation on pressure. I am shooting in the dark here, do you have residual valves in the system and are they plumbed in the right direction. proportioning valve plumbed correctly and working properly with pressur exiting to the correct location

 

OK.
Bench blead the Master. Took another vacuum reading and have 20" available at the port.

One thing that I have noticed is that there is no progressive resistance when pressing the brake pedal. Basically I do not begin to get braking force until the pedal is almost at the bottom of the stroke.

 

Do you feel like the booster is providing any assist or is it hard all the way through?

I thought I had a booster problem on a car. The booster had a rubber cap on the back side (where the pushrod from the pedal went through). That was blocking air from being able to enter where the pushrod goes in and I wasn't getting any booster assist.

Once I ripped off that rubber cover, the booster provided the proper assist and I had great brakes.

The new covers have holes in them to allow air to get through.

 

I have verified that the front calipers are applying pressure and releasing. Also the rear drums are clamping and releasing.

The system has a porportioning valve like this:




This is a combination valve. There are no other residual valves. I have verified that the lines are installed correctly.

 

Just the opposite. I do not feel any progressive resistance until the pedel is almost to the full down position.

I'll have to check that.

I did try the brakes with the vacuum line disconnected and they were much worse. So the booster is helping some. Based on this test, I would think the problem has to be in the master or portioning valve.

 

Check your pedal ratio, commonly a 6-1 or 5-1 ratio is most often used when you change brakes. I use 5-1 for power and 6-1 for manual

http://www.mpbrakes.com/uploads/documents/pedalratiopdf.pdf


http://www.hotrodheaven.com/tech/brakes/brakes6.htm

 

I had a customer's Cadillac once that drove me nuts. I was managing a 'Safety Center', a lady came in and said she couldn't stop the car unless she used both feet.
She went on to say that she had had the brakes 'relined' here, at our shop.
I checked operation of brakes, drove the car, and condemned the power unit. A rebuilt unit (complete with M'Cyl) was put on the Greyhound bus from "Rare Parts", arrived the next day.
Installed the unit, drove the car: same as before.
Long story short, the owner of the shop had done a brake job on this Caddy a few months prior. Knowing his 'commitment' to a fast buck, I checked the pads he had installed....WRONG PADS!
The shop bought a new set of pads, and the power unit. (the original had been sent beforehand, as a 'rare core'...)
Check the pad-to-rotor and shoe-to-drum. (I had a set of Bendix and Raybestos shoes on my '55 F100 that I couldn't stop with both feet. Pulled them off and had them relined at local Friction Materials. (new lining is 'pink'. Anybody familiar with it?)
Presently available riveted-and-bonded lining has been called "Chinese Linoleum". The EPA has been called something else.

 

Shoes backward? Vaccuum line collapsed. I know you said you had vacuum, but at the booster? pull the line off at the booster, motor should stall

 

I would start with a pressure gauge connected to the master cylinder to see what the pressure actually is. Try it without the booster then with the booster. You can also check at the caliper.

If you are getting good pressure and the calipers are clamping, you may have the wrong pads. High end pads are metallic or ceramic and require much higher clamping force. I usually buy the cheap soft pads. They grab the rotor better requiring less pressure. The softer pads don't wear the rotors as much. You can glaze them if you drag the brakes but if you hit the brakes and get off of them, they're fine.

 

I measured the vacuum by pulling the line right at the booster check valve and plugging in my gauge. Hose is heavy wall and in good shape.

 

Perhaps you have a disc/disc or drum/drum master cylinder. Or as already mentioned, the wrong size bore in the MC.

I think you are on the right track, MC or proportional valve is the problem.

 

jrocco,
You've done a good job trouble shooting, but there are several things that haven't been confirmed, among them the master cylinder bore size, and pedal ratio.

What was the push pin (rod) length? There should be .030"-.040" clearance between the booster push rod and master cylinder primary piston bore, and this is measured with full (20"Hg) vacuum in the booster.

With the systems open, does the pedal bottom out the master before it hits the floor?

I would get rid of the combo valve, although that's probably not your problem. An adjustable prop valve is a better, simpler and cleaner choice, IMO. Adding a 10 lb residual to the rears will help reduce rear displacement and is necessary with most pre-70s drum brakes.

 

V8 Bob

I believe you are on to the problem with the piston bottoming out, which is directly affected by the pedal ratio and rod adjustment. According to his first post the master booster combo from speedway says it has a 1 inch bore. I posted him a couple sites that give him the pedal ratio formula, so maybe he is figuring that now. Hopefully he solves this problem and posts results so we all can learn something from this

 

quick note... based on the proportioning valve you have there....

if you dont bleed the brakes in proper order, the valve may move and cause some headaches.

you have to start at the right rear, then left rear then right front then left front.

i know its remedial, but thats just the way those valves are designed.

you wanna let all the pressure out of the system by cracking all of the bleeders and let the valve find its home. then bleed the system.

all that said, we've replaced a few of those valves.

 

Would have been nice if you gave us some info on the brake components. I assume you are using the stock chevy brake pedal so my guess is you have the wrong master cylinder bore. If you have the big piston GM calipers ypu would use a 1 inch bore m/cyl. If they are the little GM metrics you need a 7/8. With the stock pedal this should yield about 3 to 3-1/2 inches of pedal travel.

 

The "valve" you describe is the pressure differential switch, which moves off center when there is about a 400 psi pressure difference between the primary and secondary systems, turning on a warning light in the dash of older production vehicles. These are part of the combination or "combo" valves, which can include proportioning and metering valves. The best thing to do is not use them, new and especially used, unless you are replacing one that came on your production car. A simple adjustable prop valve and residuals, if and when needed, are all a custom brake system requires. Fact is, many do not know what the combo valves consist of, or what their functions are.

Bleeding a dual hydraulic system can start at any wheel, as there are two systems. But it does help to know how the systems are split, i.e., vertical, cross, "Y", etc, so you can bleed one system at a time.

 

Your master cylinder may have residual valves built in. You might try bypassing the combination valve to see what happens.

 

It will be interesting to see what the problem was.

 

Start the engine and run it for 2-3 min. Shut it off, wait 1-2 min. and remove the check valve from the booster listening for a "WHISH" If it does not, tou have a bad booster.
I just went through this with a booster set up for a 55 Chevy and that is what I had for a problem. I would have to press real hard on the pedal to stop. At idle, when I stepped on the brake, it would effect the engine like a vacuum leak.

 

Thanks for all the posts. Here is the latest.

I replaced the master. It is a Corvette type with a 1" bore. After bench bleeding, installing and bleeding the brakes the pedal is much better.

Before I was almost all the way to the floor before getting any brake force, now I have it within the first 1/2".

However, the car is still not stoping the way I think it should. I still can not lock up the brakes. It is better with the new master but not where I want it to be.

To answer some of the other questions.

(1) The pedal ratio is 4:1
(2) I did set the push pin to the proper length.
(3) The switch on the porportioning valve is not turning on. I have checked this with an ohm meter while applying the brakes.
(4) The booster does hold vacuum for more than 2 minutes.

If I disconnect the vacuum line to the booster (and plug it) the car will hardly stop at all. So I believe the booster is working OK.

So that leaves me with the porportioning valve or friction material of the pads.

 

Did you burn in the new pads and shoes?
Look at your brake pads , they may be glazed.