Poor Braking - Low Pressure 1971 Nova

[Novette_71]

folks,

i dont get it. i have a big problem with my brake setup, and i dont know what it is. maybe you can help.

i have a 1971 chevy nova protourer, 11" brake booster, 1" wilwood aluminum tandem master cylinder, dodge viper RT/10 calipers front, standard corvette C4 brakes rear.

big engine with low vacuum, so i have a CPP vacuum pump that starts working at 15" and stops working at 20".

with NO vacuum i get 500 to 600 psi at the caliper when pressing the pedal really hard. this is the value my excel sheet tells me i should have, so it is okay to this point. with the pump activated i get 1000 psi pressing the pedal really hard .... the difference between "with" and "without" brake booster assistance is too low. theoretically by calculation the booster should give me 900 psi alone ... it means on the road you have to press the pedal really hard to have stopping power.

also i recognize, that the travel of the pedal feels long until a first amount of pressure is build up. i was bench bleeding the master cylinder several times, as well as bleeding the lines and calipers, it makes no difference.

so how is it possible my braking prower is power assisted only slightly better than without the boost effect ???


A) Master Cylinder was bled, two front ports go to front brakes, rear port goes thru an adjustable pressure valve to the rear brakes.




B) Ignore the additional rubbe hose, the CPP pump is directly connected to the brake booster




C) For brake pressure checking I use a gauge in the caliper, for seeing the value from the driver seat there is a camera connected to my PC

[Ketzer]

I have no idea what your problem is, but you have a heck of a nice set-up there!


Jeff-

[Ron Sutton]

Can you clarify for us ... did you say the pedal travels far the first time you push on it ... then shorter the 2nd time ?

[drc270]

Originally Posted by Ron Sutton

Can you clarify for us ... did you say the pedal travels far the first time you push on it ... then shorter the 2nd time ?

That caught my attention also.
I have the same m/c except 1 1/8 bore. It required more bench bleeding than any other m/c I have ever worked with, but I am pleased with the performance.
I also wonder about the compliance with the braided lines...
What is the i.d. of the braided lines? What is the i.d. of your hard lines?

[Novette_71]

the pedal always travels the same amount before you actually feel it getting harder and building up pressure.

the first 1,5 to 2 inch pedal travel you dont feel anything, then it gets harder and pressure builds up.

also, where do the bubbles in the rear chamber come from when releasing the pedal .. .? the MC is brand new ....

i was bench bleeding the whole unit to a point, when only fluid is seen in the hoses ... at that point no bubbles occured when releasing the screwdriver out of the bore. just like you see it on youtube how-tos it worked just fine.

once i installed it the bubbles came back ...

The lines are D-03, 3.17 mm inner diamater which comes to 0.124" or around 1/8". Since these lines have 2,850 psi working pressure and 12,000 psi burst pressure i have no hardlines ... the core of these lines is like plastic , not rubber ....

well , the 1000 psi is ok for me, so there maybe is no air in the system ... just the brake booster support doesnt fit right in ... maybe the travel of the BB doesnt fit the MC ? too little stroke ?

i will do some tryouts tonight and will post the latest results ...

frustrating ... thats the whole car by the way and I really wanna drive it with functional brakes ...

[Ron Sutton]

The car is beautiful !

You have 2 issues.

1. You have an air leak somewhere, most likely at a fitting or connection. It may not be leaking fluid out, but it is definitely leaking air back in. That is why you have bubbles coming back into the M/C when you let off the pedal. You have got to find the leak.

2. Those aircraft brake lines were not meant to run the whole car ... just the flex points where the suspension is. I have had veteran race drivers with very good feel ... feel the difference at the pedal in 12" aircraft lines versus 18" lines.

There is no concern of the aircraft style brake lines failing. That's not the concern. But the inner hose is a high grade Teflon tubing material. Under high pressures (1000 +/-) ... the Teflon hose DOES flex. If you have this aircraft hose on 4 corners, averaging 15" per wheel (5') ... it is a small amount. If the whole car has these brake lines ... say approximately 24-28' ... the lines are flexing ... and absorbing some of the pressure as opposed to applying it fully to the brake caliper pistons. This requires more brake pedal pressure to achieve brake pressure in the caliper pistons.

There's more concern ... because as you actually drive the car, under hard braking, and the caliper pistons experience oscillation, knock back, etc ... the lines flex & absorb this pressure fluctuation ... you will have less braking force ... and you won't feel it much at the pedal to know you have a problem.

This is both a feel & function problem. The brake pedal will always feel soft ... and you will have way less braking force than you should.

I hate to be the messenger with bad news. But, you need to replumb the car with hard lines & use the aircraft flex hose at the corners.

You are going to have a lot of fun with this sweet ride.

Best wishes.