Spongy pedal after big bolt pattern conversion

[lilcuda]

I'm having a problem with a spongy pedal after doing a big bolt pattern conversion. I have to pump the pedal once or twice to get a decent pedal and even then, the car doesn't stop like it did before the conversion. Here's the specs before and after the conversion.

Before:
Kelsey Hayes front disks, re-sleeved with stainless sleeves a new stainless pistons done by Stainless Steel brakes about 20 years ago. Stock Raybestos pads.
10 x 1 3/4 rear drums - stock with Raybestos shoes
Aluminum Master cylinder (don't know what bore size)
Wilwood proportioning valve

After:
11 3/4 Cordoba rotors with 1997 Viper calipers mounted on AR Engineering adapters in the front with locally made stainless braided brake hoses, Hawk HPS street pads
10 x 2 1/2 rear brakes from a mid 80's Diplomat - stock everything, new Raybestos parts
Same Master Cylinder & Wilwood prop valve as above

Bled the new system thoroughly, ran about 1 1/2 quarts through it.

Used the same brand silicone brake fluid on both setups. The original stuff stopped good, had a firm pedal. The new stuff just isn't the same. I have to pump a time or two to get a firm pedal and then it stops OK, but not as good as the old stuff. It should stop way better, given the increase in swept area.

Looking for ideas as to what to do next to cure it and/or troubleshoot it.

The only thing I can think of is doing another bench bleed of the m/c.

Here's a before and after picture, just to make it more interesting. FYI - both calipers are rear-mounted, I only have an old pic of the right side and the new pic is of the left.

 

[Revhendo]

To troubleshoot, I would start with possibly blocking the ports in the master and seeing what the pedal feels like. (Should go rock hard after a couple of pumps). Then connect either the front or the rear brake line to the master and then check again. This way you can start to diagnose which system to concentrate on.

 

[72BBSwinger]

Crack the crossover lines on the bottom of the calipers like a bleeder a couple times.

 

[winstoninwisc]

like rev said start at the begining mabee bad seal in master cylinder

72 isnt that what happens when you take your car down pikes peakand miss the turn?

 

[72BBSwinger]

Yeah sorta, these twin bleeder calipers can suck to bleed trust me.

 

[67cudafb]

You still have air in the system. The lower bleeder on the caliper is tough to get the air out. There is a brake bleeder that pushes the fluid up to the master that should fix this problem. Be sure the rear brakes are adjusted properly with a slight drag.

 

[pishta]

I agree with rear brakes out of adjustment causing a spongy pedal. Tighten to allow 1 revolution of freewheel spin only, then hit brake and check again. Theoretically you can bleed (add from the bottom) by raising the reservoir of a brake supply "I.V. bag" and letting it push the air back up into the M/C. Never had a bottom feeder but a brake guy told me it had been done on a little more advanced level, ie. pressurized.

 

[67Dart273]

Combination of air and drum adjustment.

To adjust rear brakes, tighten them up good until the wheels won't turn. Normally you would back them back off until they turn with "just a scrape."

But in this case, I'd tighten them up and LEAVE them until you get them bled, and get a good pedal.

I don't know anything about Viper calipers and cannot see where the bleeders are. The "simple theory" is that the bleeders MUST be in the top of the caliper, with no "air traps" in the way that they are mounted.

What procedure are you using to bleed? If you are using a manual pedal method, it is IMPORTANT not to release the pedal before you get the bleeder closed. It can suck air back in on the return.

 

[prrc]

You are right the only way to bleed this type of caliper is with a Phoenix injection system. Thats all we use in our shop.It forces the air up the line and out the master cylinder.

You still have air in the system. The lower bleeder on the caliper is tough to get the air out. There is a brake bleeder that pushes the fluid up to the master that should fix this problem. Be sure the rear brakes are adjusted properly with a slight drag.

 

[lilcuda]

Thanks for the ideas, guys. M/C should be fine, I would think. Was installed new a few years ago, but it's a good idea to isolate it and check it. I was thinking that the rear brakes needed adjustment, so I tightened them up last week. Made things a little better, but didn't totally fix it.

The calipers are kinda similar to a KH caliper in the sense that they are fixed and have four pistons. There are two bleeders, one for each side. Calipers are mounted so the bleeders are on top.

72BBSwinger - thanks for the idea on the crossover tube. Didn't even consider that. My bleeding procedure was to start with the right rear, then do the left rear, then the right front (outer bleeder first, then the inner) and then the left front (outer bleeder first).

I'll probably have to wait until the weekend to look at it.