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Low brake pedal / poor Brake performance

 
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Brockoli  



Joined: 06 Feb 2007
Posts: 609
Location: Edmonton, Alberta, Canada

PostPosted: Sun May 20, 2018 11:20 am    Post subject: Low brake pedal / poor Brake performance Reply with quote

I have a low soft brake pedal. Brake performance is poor. The car stops but doesn’t lock the tires up like it used to under hard braking. My car has the m471 5 lug brakes. Same a 931 or early 944. Here’s what I’ve done so far.

- rebuilt calipers 7 yrs ago. Slightly honed caliper cylinders. All new seals and dust boots
- stainless steel brake lines
- new rotors and PBR pads

Recently I have done the following:
- more track days
- new master cylinder. Did a bench bleed before installed. Installed last year
- flushed brake fluid. Used a motive power bleeder and manual way too
- new wheel bearings

Just finished the wheel bearings. Bled the brakes again. No air bubbles came out. Pedal felt a bit better in the garage but once I backed out and drove down the block the brake pedal was even lower than before.

When I changed the mater cylinder last year I noticed that it leaked brake fluid into the vacuum booster. I sucked out all of the brake fluid that was in there. I do not have a gasket in between the master and Brake booster. Haynes manual says there should be one. Don’t remember if there was one when I removed the original master cylinder.

I took apart the front calipers when I did the wheel bearing to inspect them. Everything looked good. Lots of brake pad left. Rotors were not grooved and did not have a noticeable lip on them. Took out the pins and sanded them and lightly greased them.

Any other ideas?
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'80 924 M471,Wideband O2, Full coilover susp,23/19 sways,Bilstiens,KLA Strut Brace,Roll Bar,Test Pipe,BBS RS, Willwood BBK

25% of a '87 944 Chumpcar http://straightpipe.ca/forums/showthread.php?t=2454

'79 924 Sebring Edition(Sitting)
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Paul  



Joined: 02 Nov 2002
Posts: 9311
Location: Southeast Wisconsin

PostPosted: Sun May 20, 2018 12:13 pm    Post subject: Reply with quote

Bad diaphragm in the booster?
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White 87 924S "Ghost"
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Fasteddie313  



Joined: 29 Sep 2013
Posts: 1497
Location: Not Detriot - NMI

PostPosted: Mon May 21, 2018 3:49 am    Post subject: Reply with quote

Make sure that when your brake pedal is all the way up that it is adjusted to where their is a bit of a gap before it pushes on the piston.. A gap like how you adjust valve lash, to ensure that the piston can come 100% back into position..
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1980 931S - LSD - FMIC - GTS fuel - hobby
95 M3 manual coupe - summer DD
06 X5 3.0 auto - winter DD
01 RS25 coupe - 90% Retired
06 MDX - bought for mom to DD
81 Rambler BBC 454
97 Tercel long retired/sitting
77 gs750 - 95 300ex - 89 blaster
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Rasta Monsta  



Joined: 12 Jul 2006
Posts: 11351
Location: PacNW

PostPosted: Tue May 22, 2018 2:26 pm    Post subject: Reply with quote

Any pulsation in the pedal? Sliding surfaces properly lubricated? Plastic slider bushing in place? Wheel bearings adjusted correctly?
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Brockoli  



Joined: 06 Feb 2007
Posts: 609
Location: Edmonton, Alberta, Canada

PostPosted: Wed May 23, 2018 1:30 am    Post subject: Reply with quote

No pulsation of the brake pedal. A bit in the steering wheel but that may be from previously locking up the tires on track or the car sitting for long periods after the tires get really warm. Usually most of the steering wheel vibration goes away after I heat up the tires again. Steering wheel vibration does not happen during braking. More related to vehicle speed.

I sanded the pins and sliding surfaces on the front calipers. Still need to take the rears off and do them.

I realized last night the front calipers dont have those little white plastic slider bushings. Only the rear calipers do. I checked a couple of websites and only the rear caliper rebuild kit comes with them? I thought that was strange but even the Porsche PET catalog only shows them for the the rear brakes (5 lug) and front calipers for 4-lug cars.

Just installed new front wheel bearings. I think they are adjusted properly but I only drove two blocks after I installed them and bled the brakes again. Didnt go far since the pedal was worse than before.

I tried to block the brake pedal last night to inspect the SS brake lines under pressure. Used a long piece of wood jammed into the seat to keep the brakes on. The SS lines dont have any noticeable ballooning from the pressure. I did notice that some of the bleeder screws seemed to be weeping a bit of brake fluid. I cleaned around the bleeder with brake fluid and wiped it with a rag to get it dry. Came back in a few minutes and there seemed to be a bit more fluid seeping out. Very minimal. Maybe a circle 1-2 mm larger in diameter that the bleeder. I tightened the bleeders a little bit more. To the point that I think they are too tight. Then bled all the brakes with the power bleeder set at 15psi. The brake pedal seems better.

Maybe the bleeders arent sealing in the caliper? Or maybe some crud has gunked up the threads? I think I am going to replace them with new, clean out the threads and then bleed all the brakes again. Hopefully the seating surface for the bleeders arent mangled.

I had a listen when pressing the brake pedal down last night. I hear a slight clicking noise when pressing the brake and another one when releasing the brake. It happens at about 3/4 pedal travel. I had a look at the master cylinder when my wife was pushing on the brake pedal. It moves slightly under braking pressure. I tried to look for firewall cracking under the dash by the pedals but didnt see anything. Maybe this is just fire wall flex? How much do cracked firewalls move?

Any other ideas?
_________________
'80 924 M471,Wideband O2, Full coilover susp,23/19 sways,Bilstiens,KLA Strut Brace,Roll Bar,Test Pipe,BBS RS, Willwood BBK

25% of a '87 944 Chumpcar http://straightpipe.ca/forums/showthread.php?t=2454

'79 924 Sebring Edition(Sitting)
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Rasta Monsta  



Joined: 12 Jul 2006
Posts: 11351
Location: PacNW

PostPosted: Thu May 24, 2018 8:14 am    Post subject: Reply with quote

Use high temp disc brake grease on all moving parts. Recheck wheel bearing adjustment. Hub wobble can push pads out and give soft pedal.

Do the brakes pump up?
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Brockoli  



Joined: 06 Feb 2007
Posts: 609
Location: Edmonton, Alberta, Canada

PostPosted: Thu May 24, 2018 9:05 am    Post subject: Reply with quote

Yes, I forgot to mention I lubricated the front calipers when I took them off and sanded/lubed the pins. I will do the back as well. I have some Permatex high temp brake grease I will use.

After the last bleed the pedal felt good for a couple pumps then got lower and softer. I was frustrated and went in for the night. Checked them the next night to find the bleeder screws were weeping brake fluid. Bought new bleeder screws at NAPA. Took out the old bleeders and the hole at the bottom of the bleeder was crushed in. I believe I probably over torqued them at some point which caused them to become crushed and barely leak/allow air in. Then when I tightened them down more i really crushed them. With the small hole/port crushed I believe the seating surface did not mate to the surface in the caliper. Hopefully I did not mess up the seating surface inside the caliper.

I bought the wrong size of front bleeders so had to go back to NAPA. Should have them installed tonight or this weekend. Hopefully this restores my braking performance. I guess torque specs on bleeder screws is important.

Side note: all of the bleeder screws are M7x1.0 threads. The front bleeders use a 8mm wrench and the rears use a 7mm wrench but the thread size is the same front and rear. I'm not sure why Porsche would do this. The front bleeders have a 928 part number and the rears have a 901 (911?) part number so maybe thats what they had available in the parts bins at the time.
_________________
'80 924 M471,Wideband O2, Full coilover susp,23/19 sways,Bilstiens,KLA Strut Brace,Roll Bar,Test Pipe,BBS RS, Willwood BBK

25% of a '87 944 Chumpcar http://straightpipe.ca/forums/showthread.php?t=2454

'79 924 Sebring Edition(Sitting)
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Kenodog  



Joined: 25 Jun 2003
Posts: 2416
Location: Vancouver,B.C.

PostPosted: Fri May 25, 2018 4:12 am    Post subject: Reply with quote

Had this on Olive when I picked her up. Turned out to be a pinhole in the brake line letting pressure out. Finally blew halfway home in Denver, bad deal. Pump the pedal several times and look underneath for fluid.



Leigh
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1981 931, Sabine
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Brockoli  



Joined: 06 Feb 2007
Posts: 609
Location: Edmonton, Alberta, Canada

PostPosted: Tue May 29, 2018 2:06 am    Post subject: Reply with quote

The front right bleeder screw is still leaking after replacing it with new. Took the bleeder out and looked in the threaded hole. There is a bit of corrosion on the threads and seating surface. I tired a tap to clean out the threads and a small pick to clean up the mating surface but it still leaks.

Might be time for a Willwood big brake kit for the fronts.
_________________
'80 924 M471,Wideband O2, Full coilover susp,23/19 sways,Bilstiens,KLA Strut Brace,Roll Bar,Test Pipe,BBS RS, Willwood BBK

25% of a '87 944 Chumpcar http://straightpipe.ca/forums/showthread.php?t=2454

'79 924 Sebring Edition(Sitting)
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Fasteddie313  



Joined: 29 Sep 2013
Posts: 1497
Location: Not Detriot - NMI

PostPosted: Tue May 29, 2018 3:46 am    Post subject: Reply with quote

The 4 opposed piston calipers from volvo 240s with ABS bolt right up, early 90's, same brake line and everything..
_________________
1980 931S - LSD - FMIC - GTS fuel - hobby
95 M3 manual coupe - summer DD
06 X5 3.0 auto - winter DD
01 RS25 coupe - 90% Retired
06 MDX - bought for mom to DD
81 Rambler BBC 454
97 Tercel long retired/sitting
77 gs750 - 95 300ex - 89 blaster
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Brockoli  



Joined: 06 Feb 2007
Posts: 609
Location: Edmonton, Alberta, Canada

PostPosted: Tue Jun 19, 2018 9:24 am    Post subject: Reply with quote

Found a used Willwood Rennbay big brake kit for a good price. Installed the front calipers and new stainless steel lines. Brake pedal was still low and would sink with constant medium applied brake pedal pressure.

Adjusted the pedal play under the dash. Bled the whole system many times. Manually and with a motive power bleeder. Pedal is now high and very hard when the car is off. When I start the car the pedal is much lower and will sink with constant brake pedal pressure. I cannot find an external leak anywhere.

I'm guessing its an internal master cylinder leak in my new (last year) master cylinder. It was a Dorman unit but cant remember if it was remanufactured or new...

Ordered a Beck/Arnely master cylinder. it is a new unit, not reman. Hopefully this will help...
_________________
'80 924 M471,Wideband O2, Full coilover susp,23/19 sways,Bilstiens,KLA Strut Brace,Roll Bar,Test Pipe,BBS RS, Willwood BBK

25% of a '87 944 Chumpcar http://straightpipe.ca/forums/showthread.php?t=2454

'79 924 Sebring Edition(Sitting)
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