[ausgeflippt951]

Ok, I need to bounce some thoughts off you smarter-than-I gurus:

The brakes on my car ('86 951) have been oddly soft for years (since I bought it, six years ago). They have a long pedal travel that does not get impacted by pad thickness, fluid newness, etc. With the car running, it's about twice (or more) the travel it should be.

I've been tracking with my car for the entire time and while I have been religious about servicing my brakes, I can't get this damn softness to go away. I like a rock-solid brake pedal, and this most certainly is not.


I'm thinking it's my Booster, purely through process of elimination: it's original and is the only thing I haven't replaced. That said, the boosters on these cars hardly ever fail, and when they do it's a leakage that causes a slow leak of pressure when the brakes are applied. In my case, if I hold my foot on the brake, once pressure is built it doesn't continue to drop. It just takes a lot of pedal travel to pressurize.


1) Master cylinder is new
2) Brake pads are new
3) Fluid is new, and the issue persists regardless of the brand. Currently have EXP600.
4) Calipers have been replaced with New-Used units, and I felt no change in pedal application. Indicates calipers likely aren't the issue.
5) New stainless steel lines, and the hard lines are in good shape with no kinks as far as I can see.
6) The booster check valve seems to be working: I can get flow in one direction but not the other. They're cheap so I have a new one on order anyway.





Thoughts? Here's something else I've been considering but wary of: perhaps the brake pedal clevis needs to be adjusted? The pedal travel with the car off (non-boosted) is pretty short, so I'm hesitant to fool with the brake pushrod.


Thanks in advance!

[roundel]

I have a friend that droves a Merc Cossie. He had a persistent soft peddle. I repeatedly asked if he was SURE there were no bubbles. While we were bleeding the old skool way with me on the peddles telling him it feels like bubbles, he noticed that the dual chamber reservoir did not fill both sides unless you nearly flooded it and waited for the sieve to empty. We started over and his brakes were rock solid. He was a bit embarressed.

Lesson: make absolutely sure the reservoir is FULL and NO bubbles are coming out when you bleed your brakes.

I hope your issue is this easy to rectify.

[BobNovas]

I just invented something (in my head) that might help - what about sticking a paper-thin (0.008") force measurement gauge (http://www.tekscan.com/load-measurement-system, B201-H, 1-1000lb) between the pad and the caliper and see what it says as you depress the brake pedal? If the brake pedal is moving but the gauge is not seeing more oomph, then there's a problem.

[Fairfax 944]

Booster has to do with the pedal force, and should not affect pedal travel.

Are you use stock calipers, or a caliper from a different car or aftermarket?

Are you using the right model of master cylinder?

My thought on these two questions is everything could be working properly, but your calipers/master cylinder are not matched. Some brake master cylinders put out more volume than others.

- - -
Other thought: you push the brake pedal once and there is a lot of travel. What happens when you immediately release and push a second time? Is it then rock hard or feel spongy?

Does this happen when the car is moving, particularly right after taking a hard turn, or does it act this way when the car is stationary. If it happens only when it's moving, the pads could be getting pushed back by the rotor, which is not uncommon in some cars on the track.

[VaSteve]

Check for firewall cracks as well.

[CanAm]

The brake pedal on the Cayman is on the soft side, which has been exacerbated by overheating issues, but two things that have helped are good bleeding of the brakes and upgrading the master cylinder to a GT3 master cylinder.

[ausgeflippt951]

Originally Posted by roundel

I have a friend that droves a Merc Cossie. He had a persistent soft peddle. I repeatedly asked if he was SURE there were no bubbles. While we were bleeding the old skool way with me on the peddles telling him it feels like bubbles, he noticed that the dual chamber reservoir did not fill both sides unless you nearly flooded it and waited for the sieve to empty. We started over and his brakes were rock solid. He was a bit embarressed.

Lesson: make absolutely sure the reservoir is FULL and NO bubbles are coming out when you bleed your brakes.

I hope your issue is this easy to rectify.

Thanks Bob#1 -- I ran this same conundrum by my buddy and he had the same input. I use a Power Bleeder every time; when I replaced the MC the brakes felt the same as they did before...perhaps a teensy bit better.

Originally Posted by BobNovas

I just invented something (in my head) that might help - what about sticking a paper-thin (0.008") force measurement gauge (http://www.tekscan.com/load-measurement-system, B201-H, 1-1000lb) between the pad and the caliper and see what it says as you depress the brake pedal? If the brake pedal is moving but the gauge is not seeing more oomph, then there's a problem.

Thanks Bob#2. Interestingly enough, I've used these to set up race cars -- only way to make sure you're getting the right brake balance without having to go out on track. Alas, I don't personally own a set...

Originally Posted by Fairfax 944

Booster has to do with the pedal force, and should not affect pedal travel.

Are you use stock calipers, or a caliper from a different car or aftermarket? Stock 951 calipers all way 'round. All in good shape with no leaks.

Are you using the right model of master cylinder? Stock ATE OE replacement.

My thought on these two questions is everything could be working properly, but your calipers/master cylinder are not matched. Some brake master cylinders put out more volume than others.

- - -
Other thought: you push the brake pedal once and there is a lot of travel. What happens when you immediately release and push a second time? Is it then rock hard or feel spongy? This is a great question and one I should've provided in my original post: when I first apply the brakes, there's gobs of travel and the pedal feels like shite. Then immediately apply the brakes and the travel is reduced by ~half. What does this point to?

Does this happen when the car is moving, particularly right after taking a hard turn, or does it act this way when the car is stationary. If it happens only when it's moving, the pads could be getting pushed back by the rotor, which is not uncommon in some cars on the track. It happens all the time, including when stationary. I have gotten into the habit of doing a little dab on the brakes when going down the straightaway on the track but this really is a problem that doesn't seem to be a function of pad push-back.

Good questions; see in-line above.

[Fairfax 944]

Originally Posted by ausgeflippt951

Other thought: you push the brake pedal once and there is a lot of travel. What happens when you immediately release and push a second time? Is it then rock hard or feel spongy? This is a great question and one I should've provided in my original post: when I first apply the brakes, there's gobs of travel and the pedal feels like shite. Then immediately apply the brakes and the travel is reduced by ~half. What does this point to?

You could be compressing air on the first pump, and once the air is compressed, you can apply sufficient pressure.

You could be pushing the pads against the rotor on the first pump, and then all is well after that first pump. Sometimes, rebuilding a caliper will result in the movement of the piston inward, due to the newness of the rubber seal. But, you said this has gone on for a while, so it's probably not the newness of the calipers. This leads back to the possibility of air in the system.

When you installed the new master cylinder, did you bench bleed it before installing? If so, are you sure you did it right? When I put a new brake master cylinder in my 951 this summer, I was sure I did it right, but the test drive revealed air in the system, and I had to bleed it a couple of more times before it was solid.

[Fairfax 944]

One other question: Do you have a brake pressure transducer in your system to measure brake pressure for a data logger? If so, you need to be careful to make sure the air is out of the system at the point the transducer connects to the brake lines.

[ausgeflippt951]

I did bench bleed the MC before installing, but I suppose I could re-bleed it. Nothing I like more!

As an extra data point, what was the procedure you used to really bleed it right? I could be doing it wrong (always a possibility).


So, sounds like the consensus is that it's likely not the Booster.