thewheelerZ Posted August 7, 2019 Report Share Posted August 7, 2019 Any help would be appreciated before I just throw money at parts. We bled the brakes and just rebled to make sure. All good for fluid/air. No visible leaks in the brake system. When the car is off the pedal is nice amd firm like normal and won't go to the floor with lots of solid foot pressure. When the car is running the pedal goes really soft and we can't get a nice firm pedal. When we disconnect the vacuum hose to the booster the pedal goes nice and firm again. Is the booster "over powering" a failing master cylinder (internal seal or something?). It seems to me that a failing booster would Simply mean less boost on the pedal which would mean a soft pedal again...? Are we dealing with a failing master cylinder? What else am I missing? Quote Link to comment Share on other sites More sharing options...
wvumtnbkr Posted August 7, 2019 Report Share Posted August 7, 2019 The booster just helps you push on the pedal. A failing booster would make the pedal harder, not softer. I don't necessarily have an answer for you because that was an issue I chased for a long time. I replaced the master twice, all of the hoses, and the calipers. Nothing worked. The car got totalled (not due to brake loss) having never solved it. Check for bulging lines when somebody is pushing on the brake pedal. 1 1 Quote Link to comment Share on other sites More sharing options...
Richard Posted August 7, 2019 Report Share Posted August 7, 2019 It could also be a very small leak in the line somewhere that is all but unnoticeable. I've had a bad MC before so my first question would be; Is the MC factory or just an aftermarket replacement? Quote Link to comment Share on other sites More sharing options...
rod rammage Posted August 7, 2019 Report Share Posted August 7, 2019 A bypassing master cylinder will allow the pedal sink to the floor as you continue to press HARD on it . You weren't clear if this is the case? Quote Link to comment Share on other sites More sharing options...
thewheelerZ Posted August 7, 2019 Author Report Share Posted August 7, 2019 8 minutes ago, rod rammage said: A bypassing master cylinder will allow the pedal sink to the floor as you continue to press HARD on it . You weren't clear if this is the case? With no booster (car off and/or booster vacuum disconnected) push almost as hard as I can. Hard pedal. With booster, medium pressure puts it nearly to the floor. Quote Link to comment Share on other sites More sharing options...
Team Infiniti Posted August 7, 2019 Report Share Posted August 7, 2019 (edited) 23 minutes ago, rod rammage said: A bypassing master cylinder will allow the pedal sink to the floor as you continue to press HARD on it . You weren't clear if this is the case? More then one way to skin a cat, sometimes this is not going to show the issue. The way I diagnose a bypassing master is to hit the brakes medium hard like approaching a late traffic light then ease up just enough as to hold the car idling in drive(or just enough to keep it from rolling) wait just like that up to 2 minutes, if bad, the pedal will drop with no change in foot force. Worn master cups may seal better when applied hard, we had a lightly bypassing master on the racecar for quite some time before it manifested enough to notice low pressure on the rear circuit causing accelerated front wear. 1 hour ago, wvumtnbkr said: The booster just helps you push on the pedal. A failing booster would make the pedal harder, not softer. This, the problem described is not a booster fault. 1 hour ago, wvumtnbkr said: Check for bulging lines when somebody is pushing on the brake pedal. This Connect booster, start engine, have someone pumping the pedal and watch for a bulging brake hose, a normal line will squirm a little but not visibly bulge More history needed, did this issue come about while being raced or was work being done to the brake system when the problem arose? Edited August 7, 2019 by Team Infiniti Quote Link to comment Share on other sites More sharing options...
thewheelerZ Posted August 7, 2019 Author Report Share Posted August 7, 2019 7 minutes ago, Team Infiniti said: More history needed, did this issue come about while being raced or was work being done to the brake system when the problem arose? Last race we had a long pedal as weekend wore on. Thought it was just knock back. Replaced two front ss lines and full brake bleed. Pedal still soft. Quote Link to comment Share on other sites More sharing options...
Team Infiniti Posted August 7, 2019 Report Share Posted August 7, 2019 1 minute ago, thewheelerZ said: Pedal still soft How are the rear lines? Quote Link to comment Share on other sites More sharing options...
thewheelerZ Posted August 8, 2019 Author Report Share Posted August 8, 2019 1 hour ago, Team Infiniti said: How are the rear lines? Perfect. Quote Link to comment Share on other sites More sharing options...
wvumtnbkr Posted August 8, 2019 Report Share Posted August 8, 2019 3 minutes ago, thewheelerZ said: Perfect. Verified by mashing on brake pedal while somebody watches? Quote Link to comment Share on other sites More sharing options...
thewheelerZ Posted August 8, 2019 Author Report Share Posted August 8, 2019 51 minutes ago, wvumtnbkr said: Verified by mashing on brake pedal while somebody watches? Yeah. 2 hours ago, Richard said: It could also be a very small leak in the line somewhere that is all but unnoticeable. I've had a bad MC before so my first question would be; Is the MC factory or just an aftermarket replacement? Factory? We didn't replace, it's a 180k mile car. Quote Link to comment Share on other sites More sharing options...
housejoe Posted August 8, 2019 Report Share Posted August 8, 2019 On certain vehicles with ABS,when the engine is running and you press very hard on the brake, the pedal will sink to the floor, generally in about 5 seconds. This can be experienced on 2004-present VW/AUDI products. I have never gotten a straight answer as to why, but the best theory I have heard is that ABS valves have some degree of leakage and the additional force applied by the booster is enough to cause them to leak down. Of course if you apply that much force to the pedal at speed you are immediately in ABS modulation and then the pump takes over.. 1 Quote Link to comment Share on other sites More sharing options...
Team Infiniti Posted August 8, 2019 Report Share Posted August 8, 2019 (edited) All the easy stuff is covered...Caliper bracket flex? Go ahead and change the master? Edited August 8, 2019 by Team Infiniti Quote Link to comment Share on other sites More sharing options...
TimS Posted August 8, 2019 Report Share Posted August 8, 2019 Pressure gauge on each port of the master, plug each port of master and check pedal, any fluid between master and booster?, plug individual lines and find line that allows soft pedal. Have to start eliminating good parts of the system 1 Quote Link to comment Share on other sites More sharing options...
wvumtnbkr Posted August 8, 2019 Report Share Posted August 8, 2019 Agreed with above. Harbour freight sells line clamps. You can use those to see which line is squishy and verify if master is bad. Hook up all 4 line clamps. Remove one at a time to see which corner is soft. If the pedal drops with all 4 on, it's the master.... 2 Quote Link to comment Share on other sites More sharing options...
TimS Posted August 8, 2019 Report Share Posted August 8, 2019 A couple brass flare plugs and you will have your answer in an hour Quote Link to comment Share on other sites More sharing options...
Black Magic Posted August 8, 2019 Report Share Posted August 8, 2019 I would bet your booster is bad.... Not that common, but sometimes a booster will fail and develop a leak through the front where the master bolts on. This can create a vacuum on the rear side of the master, which can reduce the pressure between the master cylinder piston seal and the wall of the master cylinder. You will get mystery air intrusion and soft pedal. Also possible but not common, you can develop an issue where the back of the master has no place for air to escape where the master is pushed by the booster. This can push air past the master cylinder piston seals as well. Are boosters easy to find? I would swap that and go from there. 1 Quote Link to comment Share on other sites More sharing options...
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