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Old 05-31-2021, 06:09 AM
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Default Brake pedal pressure issue

Every few years I take out the calipers, clean them and replace the piston seals & dust boots. This time went just as all the other times have gone, a little frustrating getting the pistons in but nothing that made me feel like I had to get someone else to do the job for me.

The reinstallation of the calipers again went without a hitch. Then the brake flush. I did have to recruit my daughter to depress the brake pedal because the hose on my Motive split while I was pumping it up to pressure. After two manning it I step in and the brake pedal drops the the floor. I redo the brake flush and retest, again the same condition. Brake pedal to the floor.

I do some research, go through my car and deduct the master cylinder has taken a dump. Almost 32 yrs it's understandable. I don't like it but it is what it is.

I have installed the master cylinder (hope to never have to do that again) and I flush the F*%K out of the brake system in my car. I first do a gravity bleed just letting the fluid flow, then I pull out my Mityvac & go through that process and finally use my Motive for a power flush. I must have gone through 4 bottles of brake fluid (thank God I work at a dealership otherwise I might as well buy stock in that shit). I sit in my car, say a quick "please God let this f*^$er be fixed" prayer and depress the brake pedal. There is a ton of pressure. So I figured I got. I get the wheels back on the car and put it back down on the ground. Back it out of the garage and as I'm rolling backwards I step on the brake pedal and it feels a little soft. I don't think too much of it until I come to the stop sign in front of my house and the pedal goes to the floor. I pump it and I'm noticing the pressure comes up a bit but still mostly soft. When I get back into my garage, I turn the engine off and when I depress the brake pedal again, it's got full pressure. It feels nice and tight and has very little movement, turn the engine on again and back to the original condition. It's very spongy. The only thing I didn't do in this process was flush the clutch slave cylinder.

Is it possible that the clutch slave cylinder also has air in its system which is causing the loss of brake pedal pressure when the engine is on? Please note that I did not experience and gear shifting issues or clutch engagement/disengagement while I was driving my car around my neighborhood on its test drive. What could I have missed that gives me brake pressure with the engine off but none as soon as the engine is on?

Have any of you gone through a similar condition and what did you do to correct it? Any suggestions on what I could try would be appreciated. Thanks
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Old 05-31-2021, 07:29 AM
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Air or leak. With car off and no vacuum boost it is less noticeable

Also preload the pads. Do this pad by pad all around after replacing seals. Pull the pad out and pump the pedal just enough to move the piston in a bit. Push the piston out just enough to be able to wiggle and hand force the pad in. The pad going in has to push the piston out and you have to feel drag as the pad is pushed in. Seals can pull the pistons back too much when they are new. This goes away after a while but preloading stops it right away. I have seen people go nuts with “air in the lines” when it was new seals.

Are the calipers on the correct sides of the car? Bleeder screws belong on the top
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Old 05-31-2021, 11:28 AM
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Kurt, I am at the point where I do not believe there is any air in the system. I made good and sure of that with all that I did. I believe there is something else going on.

Thanks for the suggestion. I have not done that but I did have someone step on the brake pedal for me while I looked at each of the brake pads to make sure the piston was extending to cause the pad to pinch the brake rotor. Plus when I spin each hub and have the brake pedal depressed the rotor stops, so I know the caliper is engaging to a degree.

I’m glad you made the recommendation you did because I was about to take each one of the calipers off and redo the whole seal replacement process again in the off chance one of the seals had become twisted in its groove when I was reinserting the pistons.

To answer your question, yes. The calipers are in the correct location. I’m very careful to make sure everything is back together the way it was before I ripped it apart. So much so that I even made sure the pistons are at 20 degrees like they are supposed to be.

I’ll update again after I follow your suggestion. Thanks very much.
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Old 06-01-2021, 10:32 AM
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Mike-

I chased a brake problem in my 964 for a long time plus two very well regarded shops in my area. I got a hold of the last MBC at the factory in Germany after a local dealer vouched for me that I wasn't part of Singer only to find out my rear rotors were beyond their minimum thickness creating my long pedal. I have no idea specifically what your car's issue is but please look at the basics first.

Just my 2 cents that cost me many thousands of dollars.
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Old 06-01-2021, 10:37 PM
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Mike-

I chased a brake problem in my 964 for a long time plus two very well regarded shops in my area. I got a hold of the last MBC at the factory in Germany after a local dealer vouched for me that I wasn't part of Singer only to find out my rear rotors were beyond their minimum thickness creating my long pedal. I have no idea specifically what your car's issue is but please look at the basics first.

Just my 2 cents that cost me many thousands of dollars.
MBC? Master brake cylinder?
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Old 06-02-2021, 09:09 AM
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Verify rotors are on straight, wheel bearings properly torqued, and axles tight. A little wobble can push away the pads making for a longer peddle. Sometimes a quick bleed after a test drive gets the last bubbles to rise to the bleeder too.

How old are your brake lines? Did you ever allow your calipers to dangle off the lines? I ask because I had a brake line crimp come off in T1 at Summit.

Good luck!
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Old 06-02-2021, 01:31 PM
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Quote:
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MBC? Master brake cylinder?
Peter-

Yes Master Brake Cylinder.
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Old 06-02-2021, 09:58 PM
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Quote:
Originally Posted by roundel View Post
Sometimes a quick bleed after a test drive gets the last bubbles to rise to the bleeder too.
^^^. Despite trying several methods (vacuum, pressure, gravity, wife leg workout), I always get a few more bubbles after a test drive with some spirited braking.
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Old 06-03-2021, 08:50 AM
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Quote:
Originally Posted by R717 View Post
^^^. Despite trying several methods (vacuum, pressure, gravity, wife leg workout), I always get a few more bubbles after a test drive with some spirited braking.
yep. do the same after a full rebuild and bleed. Finish brakes, flush and reset pads. finish the other stuff. Drive car and burp the brakes again. More than once a client has said the brakes Never Ever felt as hard and crisp.

I no longer bother with anything other than power vacuum bleeding. Not the hand pump Miti vac but air power type vacuum. Pressure bleeding risks the occasional hose blow or popping off a fitting and resulting fluid shower.
No chance of that with vacuum and it even lets you suck the little dribble up from the bleed fittings before putting the cap back on. Vacuum lets you safely remove the fluid from the reservoir prior to a flush.
Pressure bleeding makes the bubbles smaller, more dense and less likely to release from the fluid or trap points. Vacuum makes the bubbles larger than normal and far more likely to be entrained in the fluid and removed. Night and day difference in time spent and results in removing air. Fhysics!

Tapping the caliper with a soft mallet while flowing fluid helps too.
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  #10  
Old 06-12-2021, 05:23 AM
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Quote:
Originally Posted by roundel View Post
Verify rotors are on straight, wheel bearings properly torqued, and axles tight.

How old are your brake lines? Did you ever allow your calipers to dangle off the lines?
None of that stuff came off, I upgraded to steel braided lines years ago and I did not let the calipers dangle. Good suggestions though.

After following Kurt's suggestion of exercising the pistons I believe I may have resolved the issue.

I started with removing both pads from one caliper, then having someone pump the brake pedal to allow the pistons to extend out and then retracted them back in. I did this five times each caliper. Then I removed only one pad from a caliper and went through that same process of having someone pump the brake pedal and then pushing that piston back into the fully inserted position. I did this five times, put the pad back in and removed the next pad, and did it all over again with the next piston. On the last cycle I had my helper hold the brake pedal while pushed back against the piston to verify there was pushback that could be felt through the brake pedal while I pressed the pistons back into their resting position. I went through the same procedure until I finished all four calipers. I then proceeded to bed in the brakes, starting at 40mph down to 10mph for 5 brake cycles, then 60mph to 10mph followed by a series of 80mph to 10mph brake cycles. The way my car's brakes feel now is so much more like it felt before I redid the caliper seals. I haven't had a chance to take my car on a proper drive yet but hopefully I'll get a free couple of hours this weekend to test it and recheck my work.

Thank you again for the suggestions and input on what I should try doing to fix the low brake pedal issue I was experiencing. It is genuinely appreciated.
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