Brake Question

[Dave Forster]

I am having a problem with the brakes and am wondering if anyone might have seen something similar, or have a solution?

My car has the Tilton floor pedal assembly with balance bar, Brembo brakes and (currently) 5/8" Wilwood Master cylinders. There are about 100 street miles on the car and brake system.

After spending hours bleeding the brakes with the old school up/down method, a vacuum bleeder and a pressure bleeder, I was pretty sure that all of the air was out of the system, but the brakes were still "squishy", with little resistance through most of the pedal movement.

To isolate the problem, I pulled both the master cylinders off the brake pedal and tried pressing the Master cylinder plunger by hand. The rear was firm, but the front soft.

I then capped the outlet from the front Master cylinder. It allowed maybe 1/8" movement of the plunger before feeling very solid. I think this eliminated the Master cylinder as the source of the problem.

I then capped the line where it enters the brake calipers on both the left and right front calipers. Again, maybe 1/8" of movement of the plunger before feeling solid (good!). I think this eliminated any concerns about air trapped in the lines between the Master cylinder and calipers.

I then reconnected and bled the front right caliper. Maybe 1/4" of movement of the master cylinder plunger before feeling solid (pretty good - some extra movement was expected for the firming and relaxing of the pistons and pad against the rotor).

I then reconnected and bled the front left caliper (inside and outside bleed screws). This time, maybe 1" of master cylinder plunger movement by hand before things firmed up. An assistant noticed that one of the pistons on the caliper was not even touching the brake pad when I was pushing the Master cylinder plunger in by hand.

It appears that the squishy brakes might be caused by uneven movement between the four pistons in the caliper. My theory is that if the pad is not being pressed evenly against the rotor, the pad may be rocking slightly and causing excessive spring-back. This would explain why the pedal moves so far with relatively little resistance before firming up.

I then removed the pistons from the left caliper. They all look fine. Having never opened up a Brembo system, I was a bit surprised to see no O-rings; what I thought was the dust cover seems to be the actual sealing surface against the pistons.

I'm a bit worried that I haven't found a smoking gun. Is there something I should be doing to help address a sticking Brembo piston? Should the seals be replaced? Should I just put it all back together again and see if the problem has magically gone away?

Any advice gratefully received. . . .

Thanks!

 

[Cam]

Dave - there should be o-rings in there, they have a square cross-section, but they should be there.

 

[Mark B.]

Hi Dave,
The brembo calipers *do* have O-rings on the pistons separate from the dust seals. Your piston should have had a groove where the o-ring seats. I disassembled mine when I had them powdercoated and all pistsons, front and rear, had O-rings. If yours are missing even one, that would be your problem

 

[Roger Reid]

No mention of brake fluid on the floor from missing o rings. Could you have missed seeing them?

 

[Shane]

If you've already driven the car 100 miles and that brake piston still hasn't come out far enough to contact the brake pad, I'd say you have a faulty caliper on your hands. As Roger said, if you were missing o-rings, you'd have brake fluid all over.

 

[Dave Forster]

You guys are right, of course. There is an O-ring inside the cylinder bore. I didn't see it behind the dust cover.

I tested again with the left line capped at the caliper and the right caliper connected. The brake pedal moved about 1/4" and then was very firm.

I pulled the pistons out of the left caliper. The looked pretty good, with only minor marks on them, so I polished them with some 1000 grit sandpaper, cleaned them up and re-installed. I removed both bleed screws, filled the caliper with fluid, then re-installed and pressure-bled again. All four pistons now appear to be pressing hard on the pad, but the brake pedal is still very squishy with both calipers connected. I can push the pedal through almost full travel by hand. I tried unbolting the caliper so it was free to ride on the rotor without any effect of being bolted to the frame, (to eliminate any twist in the mounting that could be causing excessive spring-back). Same result. . . sigh.

 

[Cliff Beer]

Dave, sounds like your front Brembos/discs are the problem.

One question - after bleeding and driving the car, a) are the discs centered in the throat of the calipers? b) are the pads properly seated on the discs?

If the discs aren't centered that's not the end of the world - the pistons will extend/position themselves to the disc, but it's better to have the discs accurately centered.

If the pads are not seated then you'll have some softness in the pedal. Without pressure in the system the pads will back away from the disc by a matter of thousands/inch if seated properly. This very small gap allows for some lateral run-out of the disc and a thermal "air gap" so that the calipers cool when pressure isn't applied.

The next question is - is your MC bore appropriate for the total square area of the pistons (both front and rear in total)? Too small of a bore will cause a soft pedal.

Just my $.02.

 

[Randy V]

A picture is worth a thousand words...
Are you calipers oriented properly?

 

[Ken Roberts]

3/4” front and 7/8” rear are the standard sizes for the brakes and 3/4" for the clutch. Dave mentioned he has 5/8” masters currently.

 

[Johan]

Dave, I'm not totally sure of the effects of it, but have you checked your residual valves? I know CamT had them marked wrong, the arrow was pointing opposite direction of flow. If that is the case fluid will flow back to MC and might give a too large play between pads and disc.
Regards Johan

 

[Dave Forster]

Thanks for all the suggestions, guys!

Cliff: Near as I can tell from reassembly and visual inspection, the pads are centered in the calipers. There is an almost imperceptible movement of the pads as the pedal is pressed and when the brake is released, both pads appear flush to the disc and there is just a very slight drag when the rotor is turned by hand (using temporary nuts to hold the rotor tight to the axle). It's possible that the pads have not bedded in completely, with only 100 fairly gentle street miles. Also, given that one of the pistons appeared to be stuck prior to yesterday's disassembly, this might have affected pad bedding in. However, there is so much movement in the pedal with hand pressure that it doesn't seem like bedding of the pads could be the sole issue.

Randy: Pictures attached of the troublesome left front caliper. In one of the pictures, the spring was removed. This was done to see if it made any difference. It doesn't. What looks like rust on the caliper pins is actually a thin coating of high temperature brake lube.

Ken: Yes, I have 5/8" Master cylinders front and rear currently. This was done because even standing on the brakes, I couldn't get a wheel to lock up (the somewhat squishy pedal I had then hasn't improved with the smaller Master - no surprise). I thought perhaps by going to the smaller Masters I could get more pressure to the calipers and better braking, but obviously, that hasn't been the case. I'm considering going back to 3/4" Master on the front at least. My kit came with Wilwood 3/4" front and rear Masters, which is what I started with.

Johan: I did double-check the residual valves. They are set with the "In" side pointing towards the Master Cylinders, which I assume is the correct way? In any case, the valves are upstream of the "T's" and the big evidence right now is that if the right caliper is connected and the left one capped at the caliper end of the brake line, the brakes are fairly hard. It's only when connecting the left caliper that they get squishy, so I'm thinking the problem is either just in the left caliper or something related to the effect of the sum of the calipers plus the smaller Master Cylinder.

 

[Dave Forster]

3/4” front and 7/8” rear are the standard sizes. Dave mentioned he has 5/8” masters currently.

Ken: I'm glad you commented on this. This was news to me and it turns out I've got the wrong master cylinder connected to the clutch, with no clutch stop. Just in case I'm not the only ignorant one, I posted this: Master Cylinder Sizes

I don't think this explains the squishy left caliper, but you might have just saved me from further damaging the clutch system and it suggests I ought to go back to the appropriate sizes of cylinders on the brakes as a starting point. Possibly that's why I couldn't get the wheels to lock up from the start.

 

[Terry Oxandale]

I don't know that a 5/8" master for these calipers is out of bounds. I run a 3/4", with Outlaw 3000 calipers using 1.75" and 1.88" (44.4mm and 47.7mm) pistons using the floor mounted Tilton 3-piston brake assembly, and would say this combination is perfect. But...I'm using 12" rotors, so I have less leverage than you'll have.

My understanding is that the GM OEM calipers (Camaro SS Brembo) use smaller pistons (40mm & 44mm) than what I am using, so initially, it would stand to reason that the master should be smaller as well for the same feel.

Now...assuming your rotors are 14", then I feel confident in saying that this provides 20% increased leverage over my smaller rotors for identical sized calipers/masters, but the Brembo will have 20% less clamping force for the same hydraulic pressure than my Outlaws. All of which means the 3/4" masters would be the true equivalent...IF your rotors are at least 14". With the 5/8" masters included in that assumption, you should have plenty of ratio to lock up the brakes...but may be a little soft?.

 

[Dave Forster]

I haven't had a chance to drive the car yet, but the problem may be resolved.

Swapping the front master cylinder back out to 3/4" has now yielded a hard pedal while pushing by hand. Of course, the real test will be driving and testing deceleration or maybe even locking up a wheel.

3/4" (on both masters) is what I started with and couldn't lock up the brakes when driving the car. However, the pedal feels harder by hand now than it did before.

The evidence isn't very conclusive, but I suspect either a problem with the smaller 5/8" cylinder trying to drive two front calipers (i.e. not necessarily just a problem with the left caliper) and/or the fact that one of the pistons on the front left caliper was sticking before I disassembled it.

btw: maybe this is also in the updated assembly manual (I hope to obtain a copy on Monday), but there is an inconsistency in the published Tilton information. Their balance bar setup instruction sheet https://tiltonracing.com/wp-content/uploads/2013/07/98-1250-600-Series-Balance-Bars.pdf implies that changing the length of the Master Cylinder pushrods doesn't affect the braking. However, their video How to Properly Adjust Your Balance Bar - YouTube says to set the rods to equal length and then adjust the front pushrod 1/4" longer than the rear, assuming a normal setup with bigger pistons and smaller master cylinder on the front. This makes a whole lot of sense and is something I didn't do previously, which would explain why I couldn't get the balance bar to be parallel when pushing the brake pedal and would definitely have affected overall braking (still doesn't explain why the rear wheels wouldn't lock up, though).

 

[Terry Oxandale]

The Tilton video makes sense IF it were a typical, more front weight than rear, car. My car (I don't think it should be too far off from other (mid-engined cars on this forum) uses the same exact set-up (calipers and rotors) on all four corners. Thus the delta between the compression for the front vs the rear is going to be significantly less. On paper, before completing the car, and not knowing exactly where the CG would be, this was the calculated set-up. Now that the car is finished, I've found I only had to bias the front by about 2 turns of the balance bar. 3/4" masters on everything.