Ron Pope Motorsports                California Custom Roadsters               

Brake Problem

S

sampanjohn

Member
Car is new 27 Spirit. Brake pedal is hard but the brakes work very poorly. Master cylinder is Corvette manual disk/disk, bench bled, 6 to 1 pedal ratio. Front brake calipers are 70's GM intermediate, rear calipers are 2000 S-10. Pressure at front caliper is only 300 psi so we put the gauge directly on one of the master cylinder ports. Still only 300 psi.

Any help is appreciated as I am ready to drive after 2 years construction once this "minor" problem is solved.

Thanks

John
 
I really think FatPat could answer better than anyone acording to your info.
 
sampanjohn said:
Car is new 27 Spirit. Brake pedal is hard but the brakes work very poorly. Master cylinder is Corvette manual disk/disk, bench bled, 6 to 1 pedal ratio. Front brake calipers are 70's GM intermediate, rear calipers are 2000 S-10. Pressure at front caliper is only 300 psi so we put the gauge directly on one of the master cylinder ports. Still only 300 psi.

Any help is appreciated as I am ready to drive after 2 years construction once this "minor" problem is solved.

Thanks

John
Click to expand...
You need to check the pedal ratio chart in the Brake Tech area. Sounds like to me you are running too large of piston in the master, 7/8 pistion master would work better with the 6:1 pedal ratio. Or increase the pedal ratio.
Jerry
 
Something fishy here , 300 p.s.i. divided by 6 =50 lbs pressure on the pedal ? Seems you should be getting more than that ??

dave
 
Changed master cylinder to 7/8" Wilwood. Pedal ratio is now 6.25 - 1. Brakes are still terrible. Hard pedal but pressure gauge at front caliper shows only 400 psi.

I am getting confused as I have changed practically everything and the problem remains.

Thanks for any thoughts.

John
 
Using the pedal ratio chart, the results you're seeing indicate "approx." 4:1 . I wonder if you have issue's w/ the stroke not being available? [Sorry for the poor wording, but I can't come up w/ a better way of saying what I mean] i.e. binding,lack of stroke, to short pushrod , bad angles???

dave
 
Wilwood data sheet shows 1.1" stroke. When my wife pushed the pedal I can measure 1" of throw at the master cylinder when the brake lines are disconnected.

Thanks for your thoughts.

John
 
I guess at this point the only thing to do is begin at the beginning. You have the calipers mounted bleeder screw up? the calipers are "floating" [so both pads are working]? lines & hoses are good [no kinks] proportining valve good? did you install real hard composition pads? How about show us some pictures[ another set of eyes can't hurt] Have you tried talking w/Spirit?
dave
 
Not sure how to post pictures but bleeder screws are up, pads are free, hoses unkinked. Proportioning valve has not been on my suspect list as I have low pressure both front and rear. Talked to Spirit and their only suggestion was the moving of the hloe on the pedal. Thinking at this point that I may just remove the residual valves and the proportioning valve and see what happens.

Thanks for your interest

John
 
There's a how to post pics guide in the FORUM section. you say the pads are free, but are the calipers able to move both ways[so both pads contact] . what do you have for a prop valve? Not being a smart ass ,residual valves pointing the right way? Also, there's guys on here that will post pics for you if you e-mail the pic's to them..
dave
 
Yes calipers are free to move . The proportioning valve and the residuals valves are Wilwood. I have rechecked the residual valves and they are installed correctly. When I get home I will see what I can do about photos.

Thanks

John
 
Just to throw out a little more info. The S10 rear drum brakes from the factory (I'm running a 1999 S 10) have very small wheel cylinder bores at 3/4". With your combo, you're not getting much power from the rear brakes and the small WC's would contribute to a very hard pedal. Actually, with even with a 7/8" MC you're getting less power at the WC piston than you are pushing on the MC piston due the smaller WC bore. The mid 90's Astro van uses a similar wheel cylinder that is 7/8" bore. That would help a little. I was able to modify the WC's from an 82 Chevy Caprice to fit in mine. They are 15/16" bore, but you need a lathe to do it. I'm also using a 3/4" bore Wilwood MC on the rear brakes (dual MC's with a balance bar).

The S10 WC's use a unique piston and larger interchanges are not directly available. Space for a larger WC is also limited.

Also, they typically came with metallic brake shoes which take a lot of force and heat to work right. This was done, as the story goes, to keep the rear brakes on the pickup from locking up. I went with organic replacement shoes that work at a lower temp. These are usually sold as the cheapest grade of replacement shoes.

I found ,on my car at least, a major portion of the braking was with the rear brakes. Probably due to carrying most of the weight on the rear wheels. When I got the rear brakes working as they should, the car stopped straighter and in a MUCH shorter distance.

Mike
 
I had a problem getting the disc brake front end with S-10 rear and drum brakes. After I made sure that all air was bleed and had no residual valves in the lines, just a proportioning valve in the rear line. I finally on the Master moved the front line to the rear spot and the rear to the front. This put the proportioning valve in the front line. Adjusted the proportioning to half way ( 5 turns ). Had a test drive and the brakes worked great, I can lock all four up if needed. Hopefully I will never need to. OH, one other thing is my MC is power assist with the booster on the MC.
I know having the lines switched is wrong, but hay it works great and how many people look under the car and complain about it ?
 
Did you get this problem sorted out?

Duke, I think Spirit recommends putting the prop. valve in the front brake line. After fighting constant front brake lockup on my car, I can see doing that. It's just not what a prop. valve was intended to do. Guess if it works, it works!

Mike
 
Still fighting the problem but I think I have narrowed it down. All your thoughts have been helpful and here is where I am now. Pressure is too low. I understand that I need at least 900 psi. I have only 600 (measured at both the front and rear calipers). Master cylinder is 7/8" bore and pedal is 6 - 1 so in theory I should have over 1,000 psi according to the published tables. Same results from trying a different master cylinder (carefully bench bleed). Any thoughts why my pressure is so low ?

Thanks John
 
A couple of more questions/thoughts:

How far is the m/c plunger going in to get the 600#, is the pedal to the floor or is it only moving a little bit?

If its only moving a little bit, is it possible to temporarily clamp something to the brake pedal arm thats higher then the brake pedal pad to try increasing the pedal ratio and see if that can get you more then 600#.

If you push the brake pedal down to where you can't push it anymore and you have a steady 600#, will the 600# hold if you keep your foot on the pedal over x amount of time? Or does the pressure increase/decrease?

What size are your brake lines?

For process of elimination purposes, you should start with the gauges hooked up to the M/C and make sure your getting a good pressure and pedal at that point.
 
You said your wife has been pushing the pedal. Have you tried checking the pressure with you pressing the pedal?

With a true 6 to 1 ratio and a 7/8" MC, you should have over 900 PSI with just 100 lbs. of foot pressure.

If you're getting only 600 PSI at the MC, then either your ratio is wrong or the foot pressure is too low. No offense meant.

Personally I had to go to almost 8 to 1 on my setup to get a comfortable pedal feel with drum brakes front and rear and they're self-energizing.

You may wind up having to increase the pedal ratio or decrease the MC bore size (or both) to get your combo to work right and feel comfortable to you. I know that's tough to hear, but it may be the only way. You could always add a booster as another option.

I went with a dual MC/balance bar setup on my car to cure the problems I was having. I'm running a 13/16" bore MC to the rear and a 7/8" MC to the front.

Mike
 
sampanjohn said:
Still fighting the problem but I think I have narrowed it down. All your thoughts have been helpful and here is where I am now. Pressure is too low. I understand that I need at least 900 psi. I have only 600 (measured at both the front and rear calipers). Master cylinder is 7/8" bore and pedal is 6 - 1 so in theory I should have over 1,000 psi according to the published tables. Same results from trying a different master cylinder (carefully bench bleed). Any thoughts why my pressure is so low ?

Thanks John
Click to expand...

I think you need more leverage, a longer pedal.
 
You must log in or register to reply here.

Similar threads

thebearded1
Master Cylinder size and brake issues
2
Replies
24
Views
5K
Neshkoro
Neshkoro
Rip VW
Brake setup for my T coupe
2
Replies
33
Views
52K
fletcherson
F
CrazyTed
Brakes and parking brake questions
Replies
3
Views
16K
Gerry
Gerry
ChrisK
Story Time.. (long post)
Replies
6
Views
1K
ChrisK
ChrisK
Neshkoro
Right front brake grabs
2
Replies
34
Views
8K
Neshkoro
Neshkoro

     Ron Pope Motorsports                Advertise with Us!     
Back
Top