Diagnosing poor brakes -Now at a total loss

[grahamkissack]

I have spent the past month working to improve my SC's brakes...just being able to lock them up would actually be nice. There's lots of travel in pedal then just mushy. I've bled about 20 times using my spouse, vacuum, pressure, and gravity. And tapped a hammer on all calipers to remove any bubbles. The rebuilt calipers are literally touching the edge of the discs and there's a small amount of drag so I'm sure they're seated properly.

I'm at a loss what else to do??...here's what has already been done...

replaced all rubber brake lines
rebuilt all calipers (about 2000 miles ago) and installed with bleeders up and piston at 20 degrees
new Mintex pads all around (about 2000 miles ago)
new master cylinder bench bled and installed - and cleaned/inspected booster
exercised caliper pistons in and out last week
adjusted pedal position
tested brake booster held vacuum
confirmed booster vacuum at 20 in Hg

Is there another component I'm missing?

I'm wondering about glazed rotors ... but they dont look it?
Can you screw up rebuilding a caliper?
What else is there to consider?

Thanks!

[Danny_Ocean]

Did you bench bleed the master cylinder before installing? There are dozens of threads here about similar problems/solutions...

Search "soft brakes" or "mushy brakes":

http://forums.pelicanparts.com/search.php?searchid=5560621

[grahamkissack]

Danny, yes I bench bled the MC and wrote about here.

Bench bleeding a MC was a waste of time

Most of what I put in leaked out while I was connecting the brakelines!

[crustychief]

Proper break in? Most pads require a break in period. Some high performance pads require a strict break in. Here is one method...
Bedding allows your brakes to reach their full potential. Until they are bedded, your brakes simply do not work as well as they can. If you've installed a big brake kit, changed your pads and rotors, or even purchased a brand new car, you should set aside time to bed the brakes according to the instructions below. Proper bedding improves pedal feel, reduces or eliminates brake squeal, prevents (and often cures) brake judder, and extends the life of your pads and rotors. For more on the theory of bedding, please refer to this excellent article by StopTech: Removing the Mystery from Brake Pad Bed-In.

Caution: Immediately after installing new pads, rotors or a big brake kit, the first few applications of the brakes will result in very little braking power. Gently use the brakes a few times at low speed in order to build up some grip before blasting down the road at high speed. Otherwise, you may be in for a nasty surprise the first time you hit the brakes at 60 mph.

If you have just installed rotors with zinc or cadmium plating, or if the rotors have an anti-corrosion phosphate coating, you should postpone the bedding process until normal driving has allowed your brake pads to polish the rotors clean and removed all traces of the plating or coating. If your new brake rotors have an oily anti-corrosion coating, you should clean this off thoroughly with brake cleaning spray and/or hot soapy water.

Read and understand these bedding instructions completely before starting. If you have questions, give us a call or email. Do not substitute higher speeds for the 60mph called for in these instructions. The heat in your brakes goes up exponentially as you increase the speed from which you brake. If you make repeated stops from 80 or 90mph with street pads, you will overheat the brakes and may end up having to replace pads and/or rotors.

When following these instructions, avoid other vehicles. Bedding is often best done early in the morning, when traffic is light, since other drivers will have no idea what you are up to and may respond in a variety of ways ranging from fear to curiosity to aggression. A police officer will probably not understand when you try to explain why you were driving erratically! Zeckhausen Racing does not endorse speeding on public roads and takes no responsibility for any injuries or tickets you may receive while following these instructions. Use common sense!

1.

From 60mph, gently apply the brakes a couple of times to bring them up to operating temperature. This prevents you from thermally shocking the rotors and pads in the next steps.
2.

Make eight to ten near-stops from 60mph to about 10-15 mph. Do it HARD by pressing the brakes firmly, but do not lock the wheels or engage ABS. At the end of each slowdown, immediately accelerate back to 60mph and then apply the brakes again. DO NOT COME TO A COMPLETE STOP! If you stop completely and sit with your foot on the brake pedal, you will imprint pad material onto the hot rotors, which could lead to vibration and uneven braking.
3.

The brakes may begin to fade after the 7th or 8th near-stop. This fade will stabilize, but not completely go away until the brakes have fully cooled. A strong smell from the brakes, and even some smoke, is normal.
4.

After the last near-stop, accelerate back up to speed and cruise for a while, using the brakes as little as possible. The brakes need only a few minutes to cool down. Try not to become trapped in traffic or come to a complete stop while the brakes are still very hot.
5.

If full race pads, such as Hawk DTC-70 or Performance Friction PFC01 are being used, add four near-stops from 80 to 10 mph.

After the break-in cycle, there should be a slight blue tint and a light gray film on the rotor face. The blue tint tells you the rotor has reached break-in temperature and the gray film is pad material starting to transfer onto the rotor face. This is what you are looking for. The best braking occurs when there is an even layer of of pad material deposited across the rotors. This minimizes squealing, increases braking torque, and maximizes pad and rotor life.

After the first break in cycle shown above, the brakes may still not be fully broken in. A second bed-in cycle, AFTER the brakes have cooled down fully from the first cycle, may be necessary before the brakes really start to perform well. This is especially true if you have installed new pads on old rotors, since the pads need time to conform to the old rotor wear pattern. If you've just installed a big brake kit, the pedal travel may not feel as firm as you expected. After the second cycle, the pedal will become noticeably firmer. If necessary, bleed the brakes to improve pedal firmness.

[grahamkissack]

Thanks Crusty...the new pads went in about 2000 miles ago at the same time the calipers were rebuilt. I went through a brake in period as it described in that good text you posted.

I'm kinda to the point I want to take the car down to John Walker in Seattle to deal with! Unfortunately, my location on vancouver island doesn't leave many good vintage PCAR mechanics around!

[crustychief]

Well I am at a loss as well, it seems as though you have all the bases covered. Good luck with finding the problem. The great thing about Pelican is there will be someone along that has had exactly the brake dilemma as you are experiencing and will have an answer for you.

[RoninLB]

Originally Posted by grahamkissack

There's lots of travel in pedal then just mushy.

I was never happy with my stock brakes for street use and had somewhat same routine as you. Basically it seemed pads never had a chance to heat up unless i drove like a city cab driver.

I then tried S fronts and all makes of pads for same routine. Replaced everything else including MC. All a no go.

I then installed a 23mm MC to replace the 19mm stock cylinder. It still sucked but was more doable.


and my routine for bleeding a dry system is gravity bleed, plastic hammer, then drive over crap roads for a brief time. Then go home and gravity bleed again. Meanwhile I have everything to psi bleed including a 5hp/60gal compressor but gravity is what I prefer. I feel that if you want to psi bleed use only a few psi.

ps: softer than stock street pads worked best for me

[grahamkissack]

Thanks Ron. Thinking of trying a softer pad as a next step perhaps and getting the rotors machined although they look pretty good.

[RoninLB]

Originally Posted by grahamkissack

Thanks Ron. Thinking of trying a softer pad as a next step perhaps and getting the rotors machined although they look pretty good.

The pads were mfg labeled Delux. I could get you the part number but Bill Verburg has the new correct lingo. If you could stop by over here I could lend you all kinds of pads etc to experiment with because I went 930

If my rotors and a fingernail can't be caught I'd figure they were ok

the 23mm M Cyl limits the pedal travel but costs big bucks for a MC. I think pelican came to around $300us with shipping

[grahamkissack]

Just an update here. I had exhausted all avenues and someone mentioned manually inserting the pistons into the calipers during their rebuild. When I rebuilt mine in 1994 using ATE kits from our host, the only way to get them in was pushing with a C clamp despite using rebuild grease. So having done absolutely everything else, I'm rebuilding the calipers again using FTE kits this time.

I've done one front and rear. First challenge was getting the pistons out...not sized but just held in by very tight seals so I used compressed air. After cleanup and replacing the seals, I was easily able to slide the pistons in. Perhaps I had a defective (oversize) rebuild kit?

I have new front rotors on the way and will give the Mintex pads one more try. Anyone ever machine rear rotors? I've read horror stories about bad warping / pulsation after rotor machining. The rear rotors are currently over 20mm thick with max runout at about 18mm I believe.

[JP911]

Random thought: is it possible that the arm (from the pedal cluster) that actuates the master cylinder has slipped or bent? I've read through all of your approaches and it may be that your simply not activating the brakes because the master cylinder is only being lightly engaged.

[Rev-it 84]

You probably installed the calipers correctly, but double check that the bleed nipples are on top and not on the bottom of the caliper, so not to trap air.

[Rev-it 84]

also, if you havent already, check the vacume hose from the booster to the engine. point of failure will most likely be right above the rear susp. torque tube on the drivers side.

[DanielDudley]

Originally Posted by grahamkissack

Danny, yes I bench bled the MC and wrote about here.

Bench bleeding a MC was a waste of time

Most of what I put in leaked out while I was connecting the brakelines!

Post number 6 is pretty much what I do. If you are rebuilding the calipers again, you can pre fill them with brake fluid, before mounting them. Bleeders are at the top when you mount them. (Don't laugh, it happens.)

Make sure you loosen the pipes at the MC, and let fluid seep out. Leave the hoses to the calipers finger tight, and loosen one of the back hoses. Let the fluid seep out, and go to the next rear caliper, then the fronts, one at a time. Then tighten all the hoses, and start bleeding from the bleeders, from back to front.

Do not EVER let the fluid in the reservoir go low. It must have fluid at all times during the bleeding. (Don't laugh, it happens.)

Bleed, bleed, bleed. I can't see how you could put the MC on wrong. There is air somewhere. FWIW, I never let the whole system drain dry anymore. I have had too many hassles in the past. This is especially critical when you get to ABS.

[petevb]

How about getting a brake pressure gauge and checking pressure at the calipers? That would tell you if it's a problem with the pads/ calipers or with the hydraulics...

[grahamkissack]

Originally Posted by petevb

How about getting a brake pressure gauge and checking pressure at the calipers? That would tell you if it's a problem with the pads/ calipers or with the hydraulics...

Peter, yes, thats actually my next thought here if the current rebuild and new rotors come up with the same result. I also have an old set of brake pads I will reinstall to confirm its not the Mintex pads.

[James Brown]

It's so hard to troubleshoot something over the WWW. Seems you have done everything correct and not just one caliper is bad but all 4. That has me thinking it might be the master cylinder. There has been events where new/rebuilt parts have come bad from the factory (shock). I thing the pressure gage is a good next step. Also, mushiness with out leakage points to the MC.

[dshepp806]

I'd suspect that MC, too.


Best,

Doyle

[304065]

What are your exact wheel alignment settings?

[makaio]

As others have said it sounds like the MC to me. I had the same problem when I replaced mine on my old SC. I purchased a MC from a different vendor and it never worked right. After a couple months of not being able to lock my brakes, I talked with a local Porsche guru and he said I may have gotten a bad MC. SO I ordered a new one from Pelican and put it in. I didn't even bench bleed it. Only used my Motive power bleeder, and it improved my braking by a tremndous amount.