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Wilwood Brake Upgrade-Different Master Cylinder?

2K views 4 replies 4 participants last post by  sunsetdart 
#1 ·
Need some help...have a 88GTA that had stock brake system. This winter I upgraded the rearend to a strange 12 bolt S60 rearend with wilwood brakes. Got everything hooked up and bled the brakes and noticed my brake light (on the dash) would come on when testing the brakes in my garage with car on jackstands. Bled the brakes again and still got the same result. Thought maybe I wasn't pulling enough engine vacuum and would need to go out and drive the car.

Since that time I decide to upgrade the front brakes with new wilwood rotors and brakes. Got everything installed, had a buddy come over to help bleed the brakes, this time I did all four brakes again. With the engine off, I can pump the brakes up and the pedal will get firm. As soon as I start the car the brake pedal will go straight down the floor. I purchased everything from Strange, which they have a call into Wilwood about this issue, because I'm sure I'm not the first person that has come across this issue before.

I'm thinking that going from the stock OEM brakes that had a total of 4 pistons (1 piston - per caliper) to my new set up with the wilwood brakes (4 pistons - per caliper) that the master cylinder doesn't have the capacity (volume) to make my new wilwood brakes work the way that they should.

At the end of the day I really don't want to convert everything over to manual brake set up if I dont have to.

Also couple of other things to note:
* I do have a vacuum reservoir that is hooked up and worked great before I made all of the brake changes.
* I can build up 18-20 inches of vacuum and still start the car and have pedal go to the floor. (I do have a vaccum gauge in the car)
*Brake booster still holds vaccum, once I pull the check valve off the booster a you can hear air going back into the booster.

Any help would be great. Thanks.
 
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#2 ·
That car came stock with power brakes and prolly a 1 1/8" bore master which should be more than large enough. Wilwood is good about posting the size of their brake caliper pistons, so you should be able to calculate the volume and compare it to stock. When you first installed the new axle and rear brakes, the dash light was telling you that there was a pressure imbalance between the front and rear systems which to me, indicates the rears weren't properly bled. If the calipers aren't on the correct sides with the bleeders pointing up, you'll never get the air out...

Russ
 
#4 ·
What size piston were used stock and what size pistons were used in the wilwoods. You have to calculate the piston area of each and compare

Calculate the square inches of piston area of a caliper with four pistons 1.75" each.
Pi=3.1417
Radius of a 1.75" piston = .875"
3.1417 x (.875 x .875) x 2 = 4.81 sq./in. of piston area

On an single pot slide caliper you calulate 1 piston for the area, on a 4 pot fixed you calculate using just 2 pistons.

Sometimes when bleeding breaks on an older car the extra peddle travel allows the master cylinder seals to reach a part of the master cyl bore that was not used and is rusted. Then the seals are damaged rendering a once operating master cyl inoperable.
 
#5 ·
Did you run a residual valve in the system? A 2 lb valve should be run. The MC should be no larger than a 1" bore and that was told to me by a Wilwood tech.
I am telling you this because I have 4 whl Wilwood discs on my car and had the same problem. I have two residual valves, one in each line. They help prevent flow back of fluid.
Also check your pedal travel and how much play is in the pedal.
BTW my brakes are power 4 wheel disc as well .
 
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