Brake pedal soft after bleeding
Thread Starter
Le Mans Master
Joined: Aug 2002
Posts: 6,080
Likes: 3
From: Clearwater FL
St. Jude Donor '06
Brake pedal soft after bleeding
Decided to flush the fluid - I have speed bleeders. On the last caliper (LR), half way through the flushing, the pedal lost pressure and I was only getting like 1/4 the amount of fluid with each pump and the pedal was very loose until the very bottom. Seems like I felt a little "click" and it went away. I was able to finish and the pedal hasn't returned. They grab at the bottom of pedal travel and seems like all four are grabbing. I do have a bias spring that I put in and I didn't run the MC out of fluid. Seems like I somehow shut off one of the circuits in the MC... Is this possible? I have had no problems with the brakes before. 
Former Vendor
Joined: Nov 2006
Posts: 2,482
Likes: 3
From: Carmichael ca
Sometimes on an older master cylinder some rust will build up just past where the seals ride. If you push the pedal further than it normally travels, you can get the seal into this rusty area and ruin the seal. Sounds like that might be what happened.
Intermediate
Joined: Sep 2008
Posts: 31
Likes: 2
From: Springdale Pa
Also consider replacing the speed bleeders. I had one that would incrementally bleed off. It was the RR. It showed no signs of leakage, but when I replaced it, the pedal stayed firm. I autocross the car, and with the old speed bleeders I would bleed it the night before an event, and by the next day I would have a soft pedal that I had to pump. Consider putting back on the traditional bleeders.... an inexpensive place to start. Bleed them in order, closest brake to farthest.
Le Mans Master
Joined: Jan 2006
Posts: 5,621
Likes: 206
From: Orlando FL
On a chance, the proportional valve might of shifted position which could prevent proper brake pedal action. Remove the plastic sensor on the bottom, in the middle, where valve is. Look inside and make sure the "cut out part" or low part of the valve is visible and aligned with the hole. If it looks high you may need to shift the valve in one direction or the other.
As you remove the sensor, note if the bottom tang looks like it will fit into the recessed area (flat area) and if things look equal distant.
This sensor usually when activated lights a lamp on the DIP and is an indication of some sort of brake malfunction. There could be several reasons for an activation however if a wheel should lose pressure or leaks fluid the valve shifts and the sensor activates a warning of a problem. When bleeding the brakes it is possible the valve shifted.
Could be an easy fix.
As you remove the sensor, note if the bottom tang looks like it will fit into the recessed area (flat area) and if things look equal distant.
This sensor usually when activated lights a lamp on the DIP and is an indication of some sort of brake malfunction. There could be several reasons for an activation however if a wheel should lose pressure or leaks fluid the valve shifts and the sensor activates a warning of a problem. When bleeding the brakes it is possible the valve shifted.
Could be an easy fix.
Burning Brakes
Joined: May 2010
Posts: 941
Likes: 10
I have seen people do this all to often.8Valve
Le Mans Master
Joined: Jan 2003
Posts: 5,858
Likes: 7
From: Santa Maria CA
That's why it's a good idea to place a block of wood under the brake pedal to limit this travel so that you don't run the piston seal into this unused section of the master bore.
With speed bleeders you also want to use teflon tape on the threads to prevent sucking air past the threads while bleeding.
Thread Starter
Le Mans Master
Joined: Aug 2002
Posts: 6,080
Likes: 3
From: Clearwater FL
St. Jude Donor '06
Thanks for the suggestions guys. There is no problem with the bleeders. I noticed the problem start with one of them open anyways. There is no brake light on so I don't think it is the proportioning valve, but I'll check before ordering a new mc. Kind of strange how it went with one stroke of the pedal; like the seal flipped back on itself. It doesn't have that bad mc sinking pedal feel either, just grabs near the bottom of travel.
Le Mans Master
Joined: Jan 2006
Posts: 5,621
Likes: 206
From: Orlando FL
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Thread Starter
Le Mans Master
Joined: Aug 2002
Posts: 6,080
Likes: 3
From: Clearwater FL
St. Jude Donor '06
I thought for grins before I replaced the MC to bench bleed the front port of the MC and sure enough it fixed the problem. I must have drained the front part of the resivor with the fluid from the rear not having enough time to reach the front part when I was pumping the pedal. I guess there are baffles in there that prevent the fluid from moving fore and aft too qiuckly because it was still over half full when I checked.
