Brake pedal pressure
 

I've got a 1980 RX7 and not sure what the problem is, so I thought someone here might know what to do.

I've noticed inconsistencies in my brake pedal pressure. Sometimes it's real firm and sometimes it's softer. I never loose brakes, but when I'm coming off the interstate I'd like to stop before the end of the off ramp. I know it should be consistent, so something is wrong.

I put a new brake master cylinder on, plus new brake hoses and shoes/pads. But it's still doing it. Is the MS defective? Or is it the booster? What about the Proportioning bypass valve?

At this point I'm stuck. I don't want to replace everything needlessly, but I don't know how to isolate the problem.

Any ideas?

Water or air in the brake fluid can cause inconsistencies. They compress at different rates so you'll get soft and hard spots during the entire cycle of the brake pedal. I would suggest having your brakes bled and new fluid put in. Brake fluid has an affinity for H2O.

Good luck!

its not your booster. when your booster goes out, you can stand on your pedal and your car barely slows down. just like "the gray na" says, you just need to get your brakes bled and remove all the air and water in the line. good luck.

cheers

Have same prob after doing _exactly_ the same thing Devy!! (Master Cyl, new lines, replacement Booster) Been 3 yrs now of no real brake action till last 3rd of pedal, then it REALLY works! Have bought speedbleeders and am hoping a thorough (!!) bleed solves the prob.

Keep us posted!
Stu Aull
80GS
Alaska

Check your front calipers guys. I had an identical problem in my 323 a couple years ago. Changed all the hoses, master cylinder (twice!), etc and eventually figured out that the caliper pistons were getting sucked back in after applying the breaks.

I determined this by throwing the front end on jack stands, taking the front wheels off, and watching the calipers/pads do their thing while spinning the wheels and a friend operating the break pedal. At that point it became evident what was going on. After releasing the pedal, the calipers would suck back, and I'd have about a 1/2 inch of space between the pads and the rotors! No wonder I couldn't stop!!

New calipers did the trick and it's been stopping like a new car ever since.

Is there a rebuild kit out there for the calipers? Or better to buy new/reman?

There is a check valve in the vacuum line that runs to the booster. Make sure it is functioning and installed correctly. You should be able to blow through it toward the motor, but should not be able to blow through it toward the booster.

Sucking the piston back in is an interesting idea, which I've never heard of before. Any ideas what could make that happen? Just plain weird, if you ask me. :)



.

They were just old and worn. IIRC, the rubber surround was stiff and causing a lot of resistance. So its not that the hydraulic system was inducing negative pressure and pulling the piston back in or anything. More just that the whole mechanism wasn't moving properly anymore due to age/wear/corrosion?. Sorry, I should have worded that better. It really was the damnedest thing to watch happen.

Swivel - speaking for MY brake issue (above) - the problem coincided IMMEDIATELY after swapping in the parts, which would seem to suggest that its unlikely pads are a culprit in this case (??).
Likewise the check valve Kentetsu??

Stu Aull
80GS
Alaska

Stu,

So what you're saying is that you have no action for a certain amount of pedal play, then it grabs like normal? I agree that its probably not your pads. You have done a really good bleed of the system right? That can work wonders.

What I think is happening here, as anytime you have a large of amount dead space in the pedal is that there is space in the hydraulic system somewhere. That space is either in the form of air in the lines, too much of a gap forming between the pads/rotors (as in my case), or...a third thing... It's also possible you got a bum master with a funny bypass valve, I suppose. The thing with breaks is that they are a simple hydraulic system. Don't try to over-think the issue and check the basic stuff first.

If you don't get anywhere by bleeding/checking calipers, can you swap your old master and booster on to see if the behavior changes?

Also, sorry if I it sounded like I was prescribing new calipers earlier. I was just stating that these issues sound *remarkably* consistent to an issue I had in the past. Not that I absolutely knew for sure what is going on here.