abossana

symptoms:
-brake pedal slowly sinks a few inches when applying pressure with the engine running
-car stops good
-no fluid loss or external leaks.
-with engine off and no vacuum assist the pedal remains firm while holding pressure on the pedal ( no sinking)

I bled the brakes with no improvement. So i decided to replace the master cylinder and nothing changed. I bench bled the new master and used my power bleeder on all 4 corners.
My next thought was to cap each brake line where it enters the caliper one at a time and see if the problem goes away which would narrow it down to a caliper issue ?

GC996

Abossana, sorry to hear about your brakes. You probably already tried this but every time I had the same issue you described, it was either on the track when I boiled my brakes of which bleeding helped but ultimately i had to change my fluid which i started to do after every track event. Or after I changed my brake fluid and I was driving around town and had to pump the brakes a number of times to get the pedal back, of which it did come back.

For sure, it could be other issues/solutions but its typically been one of the two above for me. Hope it helps.

De Jeeper

Im wondering if u dont have an issue with the booster. I have seen threads on this issue but im sure u have searched?

abossana

All the old fluid has been flushed from the system over the course of multiple bleed sessions in the last few days.

abossana

I was thinking along this path as well. Its almost like I am getting too much vacuum assist. One thing I have not done is a clutch slave cylinder bleed. Not sure if air in that circuit could effect the brakes.

Joel987

I had a similar problem when I changed a rear caliper on my daughter's Corolla. It turned out I did not tighten the brake line to the caliper enough. After bleeding a whole large container of brake fluid the pedal still sunk in a couple of inches. I had my daughter push on the brake pedal while I was looking around the caliper and saw the slightest leak of fluid squeezing past the crush washer. It only leaked under brake pressure, as I could find no leaks prior to her pushing on the pedal. Once tightened and re-bled there was no more sinking pedal.

My bet is you still have air somewhere in the system, possibly drawing it in from a bleeder screw, or perhaps have air trapped in the ABS unit.

abossana

sounds logical aside from every component being dry and leak free but i will double check hard to see areas.

JTT

I had this recently on my truck. I bled multiple times, zero leaks, etc. Spoke to a buddy, who is an actual tech (everyone is a mechanic, until a real one walks in). Seized slider. Now, I recognize that our brakes don’t have sliders as such but do have pins. For the few mins it takes to check them, pull the wheel, pins, grease and reinstall.

abossana

thanks for the insight, i will definitely look into this.

abossana

Pulled each pin, cleaned and light grease, No change. Next step is to cap off each caliper one at a time and see if I can narrow it down to a specific caliper.

e30rapidic

I've got an extra booster if you decide to go that route. It's left over from my tip to 6 swap.

996C438

Any chance you have a brake hardline with a pin hole leak hidden by the plastic under trays ? Happened to me on my C4 . Pedal started getting soft and sinking to the floor . Nothing on the ground or at the calipers . Pulled the under trays and bingo there it was .

abossana

thanks ! I’ll let you know

abossana

anything is possible at this point but when I pressurize the system with my motive bleeder it holds 20psi indefinitely with no pressure loss.

golock911

Curious. What is the pedal height after it finishes sinking? Is it too low? What happens to the pedal height if you pump it after the sinking stops? Based on no observed leaks, I'm with either the brake booster cause or a vacuum leak from the engine. Maybe check the vacuum line back in the engine compartment. Does the ECU show any weird codes that could be explained by a vacuum leak?