need advise on brakes.

t-vicky · October 17, 2015, 08:26:14 PM

My 40 Olds 4-dr has had brake trouble since day one. Soft petal the first inch then almost lock-up. I am used to it & know how to make them work. Then the wife wanted to drive it. No way am I turning her loose in it. I put a new master cyl on . No change, New power booster, no change. Checked the petal ratio, was off a little, changed it to 4 to 1 ratio, no change. New GM style proportioning valve, No change.
This is a 1980 camero sub-frame with disc & a GM rear of about the same year. rear (shoes) It has a Corvette master 1 1/8 bore, A dual diaphragm booster, & a GM proportioning valve. I am going to a adjustable proportioning valve. Do I need a 1" master cyl ? A single diaphragm booster? I am to the point of making a big pile of brake parts in the floor & starting over from scratch. Thanks for any help.

chimp koose · October 17, 2015, 10:06:00 PM

maybe the pushrod between the booster and master is too short ? first inch movement takes up the slack between them and then the pushrod contacts master cyl cup ?

idrivejunk · October 17, 2015, 10:06:43 PM

I'm very much not an expert, but it really sounds like one side of the booster might be leaking. I'd be testing it first.

t-vicky · October 17, 2015, 10:40:38 PM

Ran a vacuum test on the hose going to the booster & got 18 lbs. When I pulled the hose off the booster you could hear it suck vacuum, so no leak. I checked the push rod when I did the petal ratio. About 1/8 free play.

wayne petty · October 18, 2015, 11:12:20 AM

Quote from: "t-vicky"Ran a vacuum test on the hose going to the booster & got 18 lbs. When I pulled the hose off the booster you could hear it suck vacuum, so no leak. I checked the push rod when I did the petal ratio. About 1/8 free play.

i would like to know which brakes lock up??? fronts or the backs..

on the GM rear end... what size drums. 9 1/2" or the less often found 11"

i would like to know if the rear shoes are installed correctly. with the shorter lining toward the front.. also check the adjustment.. by looking thru the brake backing plate slot with a flashlight.. with a screw driver.. lever the star wheel adjuster forward then backward.. if it moves more than 1/8 of an inch.. your brake shoes may be too far out of adjustment.

if your rear end uses SNAP type retainers for the rear wheel cylinders.. and the fronts lock up first.. there are actually 2 sizes of rear wheel cylinders... autozone numbers 33891 are 3/4" bore and used on 99.9% of all car and small truck applications with the snap ring retained wheel cylinders..

from 83 to 88 S10 blazers with manual brakes.. they use a AZ 33892 wheel cylinder with 7/8 bores.. this will increase the brake bias toward the rears... so if your fronts are locking up too quick.. this will reduce the need for an adjustable prop valve ...

the adjustable prop valve reduces pressure to the rear wheel cylinders. so if the fronts are locking up first.. an adjustable prop valve won't help you..

lastly... with the rear brake drums off... grab the axle flange.. lift and release... have a look at where the axle goes thru the seal.. there should be NO PLAY there. when the axle bearing wears away the axle shaft.. it allows the axle, drum and wheel to get off center from the centerline of the brake shoes. this makes it really hard to get the brake shoes in adjustment.. as applying the brake actually has to lift the car so you get a soft pedal then a grabby brake... kinda like what you describe

so... is it the fronts or rears that lock up first.. this requires some chalk marks on the sidewall.. or masking tape stripes.. either a high speed frame ability video camera.. 1,000FPS or friends standing on either side of the car as you apply the brakes to get them to lock up and they see which ones are locking first.

t-vicky · October 18, 2015, 03:07:24 PM

Thanks Wayne, I will check all that but will take a couple of days. Will post as soon as I can. Thanks again.

Beck · October 19, 2015, 11:32:20 AM

Quote from: "chimp koose"maybe the pushrod between the booster and master is too short ? first inch movement takes up the slack between them and then the pushrod contacts master cyl cup ?

Quote from: "t-vicky"I checked the push rod when I did the petal ratio. About 1/8 free play.

I have had the same problem as chimp koose replied about. If you have 1/8 free play and a 4:1 ratio you need to move the pedal 1/2" before you are moving the master cylinder piston.

The Corvette master is set up for disc/disc. You have disc/drum. BUT I recall running this same cylinder in the past in the same fashion you are using it.

I built a street rod with brake issues. I fought it as you are now. I got so disgusted the car sat for about 6 months. It didn't fix itself. I finally got it lined out.

t-vicky · October 19, 2015, 05:04:45 PM

The free play is on the push rod. 1/8 Have gotten so mad at this mess cant play with it any more its time to walk away & cool down. maybe next week.

kb426 · October 19, 2015, 08:32:43 PM

t-vicky, most of us understand. Time allows clarity!

enjenjo · October 20, 2015, 01:00:48 AM

Just chill for a few days, I have been there.

Quotemaybe the pushrod between the booster and master is too short ? first inch movement takes up the slack between them and then the pushrod contacts master cyl cup ?

There are two push rod adjustments on power brakes. The first one is from the pedal to the booster, and should have about 1/8" free play. The second one is between the booster and master cylinder. this one should be about 1/16" or .060". Most new boosters are not adjusted properly here. You can buy a tool for checking the adjustment, I made mine.

Second thing is, most of the braking is done by the front wheels, so I try to match the master cylinder size to the front brakes. The S10 master cylinder is 24mm bore which is .944", which is just over 15/16"-.937". So it sounds like your master cylinder bore is too big to begin with. So the area of your present master cylinder is .994" versus the .689 it is originally, nearly 30% too big. Contrary to popular belief, a larger master cylinder give you less pressure at the wheel, and less travel at the pedal. A smaller master cylinder gives you more pressure at the wheel, and more travel at the pedal for the same pressure applied by your foot.

I don't know the location of your master cylinder, but I have seen no mention of residual valves in your system. Drum brake S10 combo valves have a rear residual valve, but not a front one, since the location of the master cylinder somewhat higher than the caliper provides enough pressure head to hold the calipers near the rotor. If your master cylinder is mounted low, or in some cases even if it.s mounted high, the calipers can retract enough to make you loose pedal travel before the calipers reach the rotor. In addition, warped rotors, or loose wheel bearings can also cause this problem.

As stated above the rear brakes provide less of the braking force. As Wayne mentioned, bigger wheel cylinders give you better pressure. But typically what you have for rear brakes work fine because most hot rods have more tire on the rear which makes whatever brake you have there more effective, because the tire won't lock before the front ones do. So in most cases you need less brake at the rear, and not more.

I hope this helped.

t-vicky · October 24, 2015, 11:27:19 PM

OK started playing with the brakes again today. The rear end is a 68-69 Firebird with 9 1/2" drums, every thing looks & checks fine. Shoes installed right, no play on the bearings. Drums are nice & round and almost new shoes. Got it out on my dirt street & had a buddy watch. I have never locked them that hard before. The back seams to work fine. It is the front that locks. Mainly the right front. (1980 Z-28 sub-frame. Up on jacks the right turns with a little drag. The left spins easy with no drag. Have checked both brake lines & found no damage. Both seem to bleed easy. Rotors both look good & turn looking round. (no run out) Hoses look good & are fairly new. Caliper on the right is new. (Putting new on the left monday)
Have never liked GM proportioning valves. First thing I am going to do is install a T fitting on the front lines & an adjustable valve on the rear. Then replace both front lines. If that dont fix it then send it to scrap heap.
I have a dual diaphram booster & 1 1/8" bore MC. Do I need to go to a single diaphram & a 1" bore MC ? They are on the firewall.

wayne petty · October 25, 2015, 04:08:17 AM

if the fronts lock up first.. you may need larger wheel cylinder diameter on the rear.. this will increase the braking bias to the rear.. a 69 fire bird rear wheel cylinder should be 7/8 bore. your application should be an EW45873... so there are several larger sizes available. and several smaller sizes also..

other forum members who build with gm rear ends might want to copy this image to their computers..

http://i.imgur.com/8qUyCRq.jpg

at rock auto the 1" bore version is
RAYBESTOS WC37080 {#2037080} Professional Grade
Category: Wheel Cylinder and 5 bucks each..

one of the 15/16" versions is DORMAN W45999 also 5 bucks each..
the other are under 4 bucks each DORMAN W51081 also 15/16"

and its far better to do this on a paved road. when possible..

it might be worth it to order 2 of the 15/16 and 2 of the 1" versions.. try them out. thats less than 20 bucks plus shipping for all 4. could you get them that cheep locally?? i don't know..

i entered just the numbers into the rock auto part number search.. not the letters...

increasing the wheel cylinder bore size increases the surface area for the fluid to push against.. applies the brakes harder.. increasing the rear brake bias.. since you cannot really change the wheel cylinder size up front..

PeterR · October 25, 2015, 07:46:03 AM

Quote from: "t-vicky"My 40 Olds 4-dr has had brake trouble since day one. Soft petal the first inch then almost lock-up.

Check if the reaction disc in the booster has fallen out during assembly. Symptoms are: push the pedal, nothing , nothing, nothing then whoa!

t-vicky · October 25, 2015, 05:35:07 PM

What is a reaction disc?

t-vicky · February 27, 2016, 10:55:05 PM

I just noticed that I never responded to how I fixed this mess.
I put in a adjustable preprotioning valve and a T-fitting for the front breaks. Had the MC off to reach the plumbing. Started reinstalling it & noticed A gap between the MC & the booster. Very gently slideing the MC on until I feel it hit the push rod. Almost 3/16 gap. There is an adjustment in that booster. FUN FUN A bolt & a lock nut down in the bottom of that hole.
The brakes work so much better. Thanks guys...