Front calipers won't build pressure

[Olds.PhD]

68 Cutlass, stock front disc calipers, Ford Explorer rear discs, Wilwood master (1" bore) and adjustable proportioning/combo valve, 11" booster, braided stainless soft lines, new hard lines everywhere except along the frame from front to back. Everything is new over the past year.

This setup, with an old brake booster, was operational. The pedal didn't like to return all the time, so I finally replaced the booster. Unfortunately, I overlooked the pushrod. Being ~.080" longer than my old one, the master was partially engaged. Drove it about two miles before I noticed the brakes holding the car back. Managed to make it back home, but they were smoking. Fixed the booster, went to go back out and the pedal was ridiculously soft. I assumed I boiled the fluid. Turned around 100 ft out of my driveway and it hasn't left since.

I have been entirely incapable of getting a firm pedal. I have a motive pressure bleeder that works great, but I just can't seem to get the air out of the system. I've bench bled the master many times, I've even bench bled it with the combo valve attached. Block off all of the ports and the piston is firm. Put it in the car, clear fluid, no air coming out of all of the bleeders, soft pedal.

Finally bought some plugs and tried all four corners. It is both front calipers. Unfortunately, they were relatively new when I boiled the brakes, and even so, I have since replaced both and still have the same issue. Piston moves fine. I've tried everything I can think of here. Even took them off to and moved them around to dislodge any bubbles that might be hiding in there while bleeding. Nothing. Pedal is hard as a rock when I block off both fronts. Soft as ever if I block off the rears.

Calipers at their core are pretty simple little devices. What the heck am I missing?

[oleyeller]

Valve probably shifted in the proportioning valve / distribution block....preventing fluid to the front.

[Olds.PhD]

Valve probably shifted in the proportioning valve / distribution block....preventing fluid to the front.

But I am getting fluid through the front. Bleeders flow just fine. Or am I missing something?

[ZZ430]

I have been thinking about this since you first posted.

Stock GM rotors?

All that remains is the pad to rotor air gap...right?

[Olds.PhD]

Yes, stock rotors. They were new before I boiled the fluid. I can take those to get resurfaced tomorrow. I have new pads for the front, so we can eliminate that as an issue.

[Olds.PhD]

Rotors turned - no difference.
New prop/combo valve - no difference.

Going to replace the master cylinder, even though it holds firm when the lines are plugged, because there is literally nothing else to try here.

[NorCal-SS]

Any chance the caliper pistons seized when overheated? Are you able to see the pistons/pads moving the the pedal is pressed?

[Olds.PhD]

Any chance the caliper pistons seized when overheated? Are you able to see the pistons/pads moving the the pedal is pressed?

That was my thought. But I replaced both front calipers. No change. I also pulled those new calipers off the rotors and made sure the pistons moved - they do. I've even pulled the pins out and left them on the rotors to make sure nothing was binding along those pins. No change.

I'm bumping the 1" bore master to a 1.125" bore. I can't find anything specific wrong here, so I'm hoping its just that the MC bore is undersized for those big front pistons plus actual disc brake pistons out back rather than the original small wheel cylinders. Not holding my breath on that one, but worth a shot.

[NorCal-SS]

Ok, how about the front soft lines? Not sure how much heat it would take to damage SS flex lines, but might be worth replacing to rule them out…?

[Olds.PhD]

Ok, how about the front soft lines? Not sure how much heat it would take to damage SS flex lines, but might be worth replacing to rule them out…?

The lines flow really well and are firm when I put a plug at the end of them. But everything else up front is about to be new, so why not those? You're also correct, they're the only thing left that would have been impacted by the heat. Got a set on order.

[NorCal-SS]

Definitely a head-scratcher here. I’d be curious what it ends up being

[Olds.PhD]

Definitely a head-scratcher here. I’d be curious what it ends up being

Yeah, I'm grasping at straws here. A brake system isn't that complicated.

[Hotwire]

Think you'll be happier w/ the 1.125" bore master. I'm running the same in my chevelle with a junkyard hydroboost setup and love it.

As others have stated, real weird problem you have. I did away w/ a distribution block, ran front to a hurst roll control and used it to split the front lines, rear goes through an individual wilwood adjust prop valve then out to the axle. I'm not using the brake light on the dash, figured no reason to run a dist block.

[Olds.PhD]

Update. Literally everything is new at this point (again). Soft lines were replaced, new proportioning valve, new master (1.125" now), new brake caliper brackets, and I replaced the calipers with new two-piston Wilwood D52s. There was no change with that. I also purchased a pressure gauge and confirmed the issue is with the calipers. With one line plugged, and the other one hooked up to a pressure gauge, I can build 1000 psi. The rears are functional during that test. If I unplug the other line, leaving one hooked to the gauge, I can only build 800 psi. Not sure about pressure at the bleeders as the SSBC kit includes absolute garbage adapters. This is without the booster active.

I've bled with gravity, I've bled with gravity and tapping the calipers with a soft hammer, I've bled with a mightyvac, I've bled with my Motive pressure bleeder, and I've had a helper pressing the pedal. No air comes out (except with the mightyvac - around the bleed screws), but good clean fluid easily flows.

What I did notice, though, was that there appears to be pretty significant movement of the front calipers, particularly the front left. The caliper appears to be twisting as it clamps. In fact, looking at it, the pin the caliper slides on is recessed about 0.25" on the top, but flush on the bottom. The other side is recessed about 0.25". So I did some measuring from the rotor to the bracket on both sides, and sure enough, the front left is out. Even though it is brand new. Not the most elegant fix, but a pry bar with a rubber tip managed to straighten it out. Put the caliper back on, and the brakes are better! At least functional. They still aren't great and it takes a hell of a long pedal throw to get them to bite, but they do eventually bite. Wouldn't trust them to pull the car down from speed, though. I had the rears turned all the way up for that test drive to make sure I had something to stop the car.

So, still have work to do, but at least one issue has been found and addressed.

[boss_hoss]

I have fought something similar for quite a while and think I finally got mine figured out. I swapped out pretty much every part of my braking system trying to chase a soft pedal. Like yours mine had a soft pedal off the top but eventually applied adequate pressure later in the pedal throw. The only way I was able to get the pedal to firm up was by bleeding the master cylinder two ways. I started with the syringe method of bleeding the master but still had a soft pedal, so I went with the traditional method with the hoses going from the ports right back into the reservoir and sure enough I still had some air trapped somewhere in the master that had to be cycled through.

The other thing I considered trying was what Hotwire suggested which is to eliminate the front circuit from distribution block and go right from the master to the front brakes.

[Pedigry]

Typically if you have too much pedal throw your MC is too small. But 1.125 is sizable, without knowing the piston size it's hard to know if it's too small or not. Since everything is brand new and calipers would only be loosing pressure if either the O rings were shot and they were leaking (which is obvious) or there was air in the caliper. You can try taking the caliper off, sliding a piece of aluminum or steel in the caliper (same size as a rotor) and bleeding it in different positions. IOW, turn it sideways, upside down, etc. and see if there is any air lodged in there. You can also gently bang on it to see if there is air lodged in it. Most likely the problem is the MC though. If it was just one caliper that had air trapped in it, the car would move left or right under breaking. I had a bad MC right out of the box before, took me forever to figure it out. But if the fronts are equally poor on clamping power, it has to be the MC.

BTW, I just noticed you have SSBC brakes? I do as well and they suck. I have manual brakes with a wildwood MC and I have to stand on them to get them to grab. I have read alot of forum threads about them and the general consensus is that they are just not very good calipers. My car came with them so I am currenlty stuck with them until I spend the money to replace them. Mine are only 11" rotors, but hell, I have had drums all the way around that stop the car better than these. I won't resell them when I take them off, I will junk them.

But as mentioned, I would start with swapping out the MC.

Let us know how it turns out.

[toddz69]

BTW, I just noticed you have SSBC brakes? I do as well and they suck. I have manual brakes with a wildwood MC and I have to stand on them to get them to grab. I have read alot of forum threads about them and the general consensus is that they are just not very good calipers. My car came with them so I am currenlty stuck with them until I spend the money to replace them. Mine are only 11" rotors, but hell, I have had drums all the way around that stop the car better than these. I won't resell them when I take them off, I will junk them.

I don't see anywhere in the thread where SSBC brakes are mentioned? The OP originally had stock, cast-iron D52 calipers and then switched to the Wilwood D52s.

Todd Z.

[Pedigry]

I saw SSBC but after further review he was referring to the bleeder kit. My mistake. SSBC is still atrotious. lol.

[TheJDMan]

I remember back to when I was bleeding brakes on Dust Off for the first time. After a couple of weeks, every bleeding method I could find and three quarts of brake fluid it still had a soft pedal. I finally got frustrated and just walked away from the car for over a month. When I got back around to it, the brake pedal was solid as a rock. The only explanation I could come up with was that there was a persistent air bubble in the MC that eventually floated out as the car sat. As for sizing the MC, I'm running a 7/8" bore MC with C6 Z06 calipers 6 piston front 4 piston rear. The MC bore size is less about fluid volume and more about applied pedal pressure. The bigger the MC bore the more pedal pressure is required.