michael7810

I have an 87 944Turbo. It has stock calipers (132K miles on them) with slotted rotors (30K miles) and new Porterfield R4S pads. I bedded the pads properly and flushed the system with new Dot 3 fluid. During the bedding process I noticed that I could not get the brakes to lockup but since the pads were new I just thought they would grab more after I had a few more miles on them. The brakes feel normal during street driving however I just did an AX and the brakes suck under hard braking. The brakes initially grab OK but then it feels like there's a block under the pedal preventing further pressure. I was putting all the force I could muster on the pedal and they would not come close to locking up. My car does not have ABS. I tested the booster by depressing the brake with the engine off then starting it and once the engine started the pedal went down another ~1/2 inch. The brakes do not pull or squeal. The vacuum hose to the booster is a couple years old and is connected securely.
I don't think it's the pads because the car does not pull and initial braking is good. As I said it feels totally fine during street driving.

Any ideas?

harveyf

Did you clean the rotors off really well when you installed the new pads? What bedding really does is transfer some of the pad material to the face of the rotors. If some of the old pad material is still there, that can affect braking.
A simple test of the booster is to clamp off the vacuum supply line with vicegrips. If the braking does not get substantially worse, then that would implicate the booster system. I tried this on my track car once, just to see if I could ditch the booster. I found the braking to be pretty awful without the help from the booster.
I know nothing about your pad selection but if you select anything in the category of a "track" or "race" pad, you know they will not work well until they get hot. If all you are doing is AX, you really want to stick with a street pad.

Gage

These two phrases are in conflict with each other. No proper bedding process I'm aware of involves hard braking.
Stock pads from a quality manufacturer like Jurid or Textar, properly bedded, will give the best all around performance.
Yes, it can take hundreds of miles of normal driving to realize full brake potential.

michael7810

The pads I'm using are street pads and I've used them before with good results. I have around 1500 miles on the new pads so they should be well bedded by now. I'll try clamping the booster line to see how much the braking changes.
Thinking about this problem has me wondering how the vacuum booster works while the engine is getting boost from the turbo?? I understand when at idle there is no turbo pressure so there is plenty of vacuum but what about when at full throttle you lift and hit the brakes quickly...is there enough vacuum for the brakes?? Is that what the check valve in the vacuum line is for??
Thanks for the replies

michael7810

I clamped the vacuum line and it made a big difference in stopping power (ie less). So the booster is working but maybe not to the level it should be. I ordered a new check valve and will see if that makes a difference. If not I'll probably replace the booster ( not looking forward to that job, I read on Pelican that the intake manifold has to come off in order to remove the heat shield on a turbo).

harveyf

I am not specifically familiar with the Turbo. That said, on my 85, there was the "venturi" element in the hoses, a plastic Y device generally under the throttle body. My understanding is that it was designed to provide vacuum under all engine load conditions. I'm not sure why but I have seen threads where folks want to get rid of it.

You confirmed the booster is working at some level. Replacing it is indeed a bear of a project. I was brought to a complete halt once simply because the damn clip that secures the actuator rod to the brake pedal would not come apart for love nor money. Before I went down that road, I would suggest you get a new set of rotors and pads. I know, not trivial money, but a heck of a lot easier than the booster job. I just put new Textar street pads and Sebro slotted rotors on my Cayman. I did go through an accelerated bedding process (multiple gradual increasing stops) and was able to track it the following weekend. All was good with the brakes. As noted by Gage, they are designed to bed with normal driving. I just couldn't afford to wait that long. A less expensive option would be to remove the rotors and get them really clean with brake cleaner. Inspect the pads. See if they are wearing evenly. Tom at T&T Racing had an issue on a thread recently where the caliper halves were not articulating properly. Check your rotor thicknesses. The Porsche wear limit is pretty nominal. I think some folks replace with every other pad replacement. Also, are you loosing any fluid, anywhere? I would keep my attention on the calipers/discs for now.

Finally, the required energy to brake any car goes up with the square of the speed. Just a remeinder that there are differences between street stopping and track stopping.

T&T Racing

Yes,
Rebuild the calipers, the puck/piston seals are not letting the pucks release from the rotor. Also switch to Motul RBF600 DOT 4 brake fluid. I ran Porterfield R4 brake pads in 1983 944 with good success until the floating calipers seized from neglected maintenance.

michael7810

I figured at 132k the calipers are getting tired. I'm trying to figure out how a sticking puck would cause the problem I'm having. If one or more pucks are not retracting, would I be able to verify that by a dragging brake (rotating the wheels while on a lift?). Also it seems that since the car does not pull under heavy braking that all 4 calipers have similar sticking pucks? I'm certainly not against rebuilding the calipers, just looking to see how to verify that is the problem (note: I would rather rebuild all 4 calipers and replace the rotors than replace the brake booster).
Thanks

PaulD_944S2

Put the old pads back in and see how they work.
You might have defective pads.

harveyf

So we get down to you are driving a fricken turbo and you really want to baseline all the braking components back to OEM spec. OEM spec should be sufficient for anything other than hard track use. Even then, the stock setup on my 944 track car never left me wanting for more.

Let's go back a bit. What was your configuration the last time you thought the brakes were fine? What have you changed since then?

I think it is a no brainer to rebuild the calipers. There is really no downside, as with the mileage you have on the car, it's time for that job anyway.

Nobody has said in plain language but are the pads you are using a race pad? If Tom is running them, I suspect they are.

We haven't really talked about your master cylinder. In general, it has 2 internal circuits. If 1 of the 2 has failed, that would of course reduce your braking power substantially. Again, all my knowledge is with an 85 NA car but I was able to buy a replacement steel MC from Paragon for about $100. Probably a little more now but as you can see, less than the price of pads. I would also replace the flex hoses at at the wheels as a matter of course.

GPA951s

Before i rebuilt my calipers on my track car i was having the same issue.. one time i thought i was going to break the firewall i was pushing so hard.. on the track it was scary.. rebuilt the calipers.. issue resolved.. FWIW i also grind the pads on the ends so they slide in and out with little or no resistance..

bonus12

If the brakes work totally fine under hard braking on the "street," but start to fail during AX, then I think you should check the heat of the rotors after each type of drive and compare. The best method would be an infrared thermometer ($15 for a cheap one). You might find they are getting too hot. Do you ever smell them while driving?

Also, maybe you busted a welt into an old rubber brake hose, and it's absorbing all the pressure. Did you update the brake lines?

This does sound mostly like a brake pad / rotor interface issue.

T&T Racing

No longer need to pontificate on further testing. Just rebuild the calipers, clean the brake pad slide areas until you see shining metal, replace corroded retaining pins, do what GPA951s recommended. Then use Permatex Ceramic High Temp Lubricant on the retaining pins and the brake pad backing plates edges that slide. I did mine and what a difference in smooth, predictable braking and release