Help with brake pressure - Only 500PSI to the front

[Treozen]

Hey folks - been a long time.

I actually sold my '63 Thunderbird to my Uncle, and so I've not have much reason to come visit you all, but he's having issues with the brakes and I'm trying to help.

As you probably don't remember, lol, Bigbrownpilot and I used the Auto City classic kit and wrote up a technical guide for disk brakes, but the brakes (in my case anyway) have never seemed right. Recently I bough a brake pressure test kit and found that we're only getting around 500 PSI to the front calipers, and only with the car running - off we get perhaps 50PSI (yes...fifty). A shop had tried to fix the issue, mostly by making thinks worse - they added a second proportioning valve behind the first, and needless to say the brakes were basically unsafe.

To correct and baseline some things, here is what's been done, in addition to the original ACC brake kit, which uses the GM metric caliper:

1) Standard PV2 Combination valve replaced with Willwood adjustable proportioning valve. Proportioning only impacts the rear circuit.

2) Replaced the GM Corvette style master cylinder with a new one - this actually did improve things a bit, but was done just to take the M/C out of the equation. It is a 1 1/8 bore, as used with similar GM vehicles with disc / drum brakes and is the same design as called for in the kit.

3) New Wilwood 10lb pressure valve to the rear

4)Checked and reset booster push rod with proper push rod tool.

5) Brakes extensively bled

With the car off, the pressure gauge will read perhaps 50 PSI. With the car running, and thus the booster in play, it will read 500 - 600 PSI.

It seems to me we should be seeing near 600-700 PSI WITHOUT the booster, and closer to 900 - 1200 with it. I don't understand why effectively zero pressure is being generated unless the booster has vacuum. Right now, the plan is to try replace the 9" booster with a dual diaphragm 8" (dual 9 won't fit) The booster in the car now was used, and although it seems to work, perhaps it has an issue - but even if that's true, why no pressure with the car off? I have considered that the calipers might be too wide for the rotor...or better stated, the pads are too far away from the disk. There are pad spacers with the kit and I installed them at the time - is there a good gap to shoot for in terms of pad to disk spacing?
Thanks all.

Allan

[stubbie]

How much vacuum at break booster?

[NigelFarage]

Does your master cylinder/kit have a GM PV2 safety valve, commonly thought to be a proportioning valve?

It could be that the differential in volume from your front to rear circuit is tripping the valve and cutting pressure to the higher volume calipers.

I ordered the "kits' for Master Cylinder and front discs from "ABSPower Brake" looks like same corvette master and Granada/S10 brakes, but this master cylinder came with a PV2 installed. I haven't driven it yet, but am wondering if I may have this same issue once I do.

https://cdn.performanceonline.com/image ... 10-986.jpg

[NigelFarage]

Another option to rule out the PV2 is to order a "PV2 Bleed tool" from amazon and lock the PV2 in the center position.