Power Brake Problem - Could use some help

WOW I'm exhausted just reading this. I'm very interested to hear the resolution! My dad and I built a 56 we went through 3 brake set ups and the problem was resoved with the final combination of a 1 1/8" master cyl. I realize you have been through the wringer on this one and I wouldn't suggest throwing more new parts at this problem with out a definative answer first. Have you been able to get any good service from your supplier! Good luck!

 

I agree on the materiel Mike, they make cheap stuff and they make good stuff, most people not knowing any different will opt for the lower price every time. The proper lining is very important....just my 2 cent's worth

 

This reminds me of two off the wall problems I found on other cars in the past.

On one my friend is complaining of having to push so damn hard but the brakes are not responding. Check this, check that...eventually find that the pedal mount is stopping his pedal from traveling and he is pushing up against a hard stop. Disassemble, cut, file, fixed. Pedal travel restored and brakes work.

On another, guy is complaining about the shitty disc conversion he just bolted in. I test it and sure enough, the brakes are lousy. Check this, check that.....I find that instead of the caliper bolts, he used some grade 8 bolts and washers to bolt down the calipers. The piston was coming out and one shoe would drag the rotor, but no clamping force because the caliper couldn't slide. Proper caliper bolts fixed that one.

Keep us posted.....I'd like to hear what the outcome is on this one.

 

A little background on this car. It was bought by a friend and the restoration was done about 8 years ago. So I do not know whose brake components are on the car.

Today I tried foot braking the car hard, and with about 1/3 on the gas pedal I pulled right away without even a squawk out of the back tires. Basically the front calipers are not doing their job at all.

From everyting I have read, going from a 1" master to a 1-1/8" master will decrease the pressure on the pads by 25%.

From what additional research I have done, GM "metric calipers" have a bore size of 2-3/8". Based on the tech info on MP Brakes site, changing to a 2-7/8" bore caliper will increase the clamping force from 2800 PSI to 4600 PSI or a 65% increase in clamping force; and that is with 120 lbs pressure on the brake pedal with a single diaphgram booster exerting 800 psi on the master.

This car has a 7" dual diaphragm booster that generates 1200 PSI out of the master.

Here's some pretty good tech info:
http://www.mpbrakes.com/uploads/documents/optomize.pdf

I am going to see what would be involved with switching to the larger dia caliper.

I hear the suggestions about changing the pedal ratio and that is a definate consideration. However it will require a bit of modifications. Basically the pivot point on the brake pedal would need to be shortened, and to do that I would have to remove the pedal and hanger assembly, reinforce and redrill a new pivot point 2" below the one there in the hanger. I would also have to cut 2" out of the brake pedal, weld it back together, and redrill the hole to move the pivot point.

 

I did verify that the pedal is not bottoming our and there are the right type bolts in the calipers.

 

Isolate the front from the rear the front brakes should do 75 percent of the stopping power. You say the pedal feels firm. Try to figure out is the whole system weak or just the front or rear. You may have the wrong size hard line going to the rear, drums need volume not pressure and disk need pressure not volume.
Since I'm cheap I would go to a junk yard and get a MC off a fullsize truck be sure and get the cobination vale with the MC and bolt it on and see if that changes anything. It sounds like you have checked all the major things. I know there are several different valves that look the same but are different rates and hook ups.

 

If you're getting 1200 psi to the wheel cylinders and calipers, the problem has to be the coefficient of friction between the pads/ shoes and rotors/ drums.

 

Have you checked the stackup of the rotor thickness, pad thickness & caliper piston travel to make sure there is enough total material in there to clamp tight??

I'll just toss that out there. Looks like everything else has been mentioned.

 

Were you able to resolve this issue?

 

Pretty much so.

There were 2 things that contributed to most of the problems.

(1) On the Tri-5 Chevys there is a brace (pedal hanger) that bolts the booster to the firewall and connects to the dash. The guy that built the car removed this brace and just hung the pedal from a hanger bolted to the booster and firewall.

Consequently, the entire firewall was flexing (a lot) when you pressed on the brake pedal. With that much flex on the firewall, you could not keep good pressure on the brake pedal duting a stop.

(2) After I removed the brake pedal and got a good measurement of the pedal ratio, it was only 3.3:1. I had to move the fulcrum hole down 1-1/2", shorten the pedal and redrill a new hole in the pedal. I ended up with a 5:1 ratio.

The car now stops pretty good, and I can lock up the wheels. It still feels like (when approaching a stop) you want to add more pedal force than normal.

I told my friend to drive the car for a while. If he still does not like the feel, the next step would be to change the front calipers. They are the small GM metric, and Speedway offers a revised metric caliper with a larger bore that adds about 25% more braking force.