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.

[Novette_71]

Ron,

thanks for your input. I was very busy today figuring out whats going on and I have big news ...

The PTFE tubing is said to be usable for a whole car, at least that is what I got to know from the supplying company who races as well ... we will see ... routing the whole car again would be a nightmare ....

I will think about the leak thing, it really bothers me not to know whats going on.

Anyways, here's my news ....

First , I started wondering if it is the brake booster or the master cylinder causing the trouble ... 1000 to 1100 psi is good for my setup, so the question was can the master cylinder deliver, and if he can at what travel of the piston.

I prepared this setup :







The tool gives pressure to the master cylinder piston ..... I eliminated the brake booster this way. I got 1100 psi at only 16.3 mm (0.641") of piston travel .... my estimation was that the travel is much higher, indicating alot of air in the system ... 16.3 mm multiplicated by the lever ratio of 4 makes 64 or around 2.5" travel at the pedal, which was my estimation (nothing happens in the first 2 inch I said) .... I could have gone higher but was unsure about the force the tool applies to the very outer part of the aluminum casting, so I quit at 1100 ....







I was impressed that it is only 16.3 mm travel ... so I started wondering why the brake booster is not having the power .... well, now you have to keep in mind that the restoration of this car took some years and I am ashamed to say that the booster is not 11" .... lol ..... I stood in front of my car and was wondering what the hell is going on ... I was so (!) sure it is 11" ... but it is not, it is about 220 mm, so either it is 9" or a 8" ... since you have the crimping area on the very outside I could imagine it is 8" and the additional material is just housing ???

I went back to my excel list and this is the result :





Actually the middle column fits good with my results .... I could reach 700 psi, was frustrated because I estimated 1200, stepped on the brake pedal really hard, reached the 1000 psi but the ratio of leg force to brake booster force was just not right .... in the right column I estimated the second diaphragm to be smaller than the first one and still think I could benefit from a dual 8" brake booster ... which I ordered today at Summit. I cant believe that I had a small booster in front of me all the time and didnt recognize it ...

So that is the plan for now .. waiting for Summit to deliver the box to Europe and we see from there .......

What do you think about the approach ... would you say i have a 9" booster right now ?? I dont know if the inch number refers to the diaphragm or the casing.

If I just see the diameters² the ratio is 11² to 9² to 2x8² so 121 to 81 to 128 (if the dual 8" would have same diameter diaphragms), so regardless if mine is a single 8" or single 9" it is not enough ...



P.S. : one more thing about the leak ... if nothings leaks out at 1100 psi what kind of leak could that be where I suck air in ... I will check nevertheless all fittings.

[clinkster]

First, that is one sick nova! I have a 70 only hope to aspire to have one as nice as yours! Its just my humble opinion but my first reaction was that the booster is way to small, which looks like you came to the same conclusion. Also in regards to the bubbles, you might want to try to re-bleed the master. Sounds like the master still has air in it. I had the same issue on my buddies car where he thought he did a good job of bench bleeding until we put in the car and mushy/too much brake travel. Turned out he could not overcome the spring pressure by pushing with a screwdriver and only bleed half the master. I just rehooked the bleed kit up with it installed and had him push with the pedal slowly until all bubbles were gone and that cured it. Dont know if that is your problem with air bubbles or not just thought i would throw that out there. btw: i would love to see more pics of it, interior as well. nice job.

[Novette_71]

so many news .....

first, i doublechecked the vacuum hoses for the brake booster. the idea was to have the vac pump and the engine both connected to the BB (with a one way valve in between).

so whenever the engine vac is larger than the activation point from the pump (15 psi) it will help evacuate. at idle (engine 10 psi) the pump is on its own ...

BUT : sometime in the past during restoration I was smart enough to put the PCV valve with a T-fitting to the same port as the brake bosster, so behind the throttle, that was screwing up my engine vac of course .... hahaha. i dont know what went into my mind .... i saw it andf thought DID I REALLY DO THAT ??? WTF.

I fixed it. and i installed the new booster dual 8".

now THIS is amazing : my MC is connected with flexible lines. you know how normally the MC is angled a little bit in the engine bay, like a wing lets say, front is a bit up.

when i replaced the BB I made myself some room and brought the MC towards the front ... angling it a bit like a spoiler, front down, maybe 20° or so. that was rather by random, i put some towels not to scratch anything and that was the position it was placed in by coincidence. all lines stayed connected. advantage with the flexible lines setup.

when i put it back onto the new BB i opened the lid to doublecheck fluid level and wanted to bleed it again. BUT the pedal was hard from the beginning and no bubbles !!!! i bled everything again to be safe, still (thanks god) no bubbles.

since i changed nothing i wonder if the tilting to the front when the MC was loose actually helped removing the air, like self bleeding .... you have two bores in the rear chamber, with the nose facing up they can never escape and stay there, but with the nose facing down maybe they found their way out by themselves ????

anyways braking is excellent now, a good feeling