Brake Pedal firm when car is off, when engine is running, goes to floor

[nctexan]

So I just finished all poly bushing install. Brakes were fine and bled before bushing job, but I opened a line not knowing I didnt have to take a front caliper off so I re-bled everything. Pedal was hard as a rock. Then I start the car, and with the motor running, the pedal turns to mush and I can push it to the floor. Google searches have said that it's a bad booster or master cylinder. I just found it really weird that they were working perfectly before, then after this last bleed, they are crap with the car runnnig. Any one have this problem before?

If it is the booster can anyone recommend a good buy? I know driftmotion offers a $200 rebuild service but I have seen reman boosters for like $100. THanks for any information and help!

 

[IBoughtASupra]

Have you ever changed the master cylinder? Have a spare you can swap on?

 

[barkz]

boosters rarely go bad. i would try a new master first. but with a new master comes bench bleeding, and you may need to adjust the pushrod coming out of the booster that pushes the plunger on the master. i replaced mine, completely overlooked the pushrod, and spent a day pissed off and taking my master off/on about 5 times. finally remembered the pushrod, adjusted it and was fine

 

[DEFIANT 7M]

I would say master cylinder, also how are you bleeding the system? I was always told RR then LR then RF then LF engine running.

 

[chris_2jz]

I heard if you dont pump your brakes before you start car after bleeding your brakes you can mess up your master cylinder..... Im not 100% on that but im just throwing out there.

Sent from my SGH-T959 using Tapatalk

 

[Nick M]

It should be hard when the engine is off. The booster only holds vacuum for one stop in case of engine shut down. And after it is hard to move, as hydraulic fluid can't be compressed, it should feel soft when the car is started and force (vacuum) is applied. But I am not there to see. Does the car stop?

 

[barkz]

I would say master cylinder, also how are you bleeding the system? I was always told RR then LR then RF then LF engine running.

it starts with the wheel furthest from ABS block. in our cars' case Driver Rear, Pass Rear, Driver Front, Pass Front, as our ABS block is on the pass side

 

[DEFIANT 7M]

it starts with the wheel furthest from ABS block. in our cars' case Driver Rear, Pass Rear, Driver Front, Pass Front, as our ABS block is on the pass side

:thumbup:Good info, some of our cars don't have ABS mine doesn't.

 

[IBoughtASupra]

Have you ever locked up the brakes on your non ABS car?

 

[DEFIANT 7M]

Have you ever locked up the brakes on your non ABS car?

Once or twice, but in my defense it was at a SCCA event.:icon_bigg

 

[nctexan]

It should be hard when the engine is off. The booster only holds vacuum for one stop in case of engine shut down. And after it is hard to move, as hydraulic fluid can't be compressed, it should feel soft when the car is started and force (vacuum) is applied. But I am not there to see. Does the car stop?

Yes, the car does stop pretty well, not as well as when I first bled them after the AZP install and bed in the pads. It seems like there is more resistance to pedal pressure now that I drove it for the first time yesterday in like 2-3 months (I've had it on jack stands while I cut out all the frozen camber bolts and was installing all poly bushes and new sway bars).

 

[nctexan]

Have you ever changed the master cylinder? Have a spare you can swap on?

No I only changed the slave, which may be part of the problem. I know I can't expect an original component of the system to hold up if I replace another with a new part. I probably need to do this. Think it is wise to just rebuild it? DM sells a kit.

 

[nctexan]

I would say master cylinder, also how are you bleeding the system? I was always told RR then LR then RF then LF engine running.

I did not follow that precise order when bleeding, nor did I bleed any one caliper with the engine running. Perhaps I need to try this.

 

[DEFIANT 7M]

I did not follow that precise order when bleeding, nor did I bleed any one caliper with the engine running. Perhaps I need to try this.

Oh okay give that a shot. The sequence may be different if you have ABS, as pointed out by Barkz. Question is the pedal firm at first then creeps to the floor with the car running? If so you have a caliper issue.

 

[Mk3*Supra]

a brake bad brake booster would mean you have a HARD pedal.. not overly soft

and replacing the slave cylinder wouldnt affect the BRAKE master cylinder.. unless you just meant if one old component went bad then another going bad isn't weird


I'd say rebleed properly and if you still have the problem get a new master cylinder

 

[clutch]

I honestly think that it is a caliper issue. The whole fun part about that is trying to figure out which one. Good luck on it though.

 

[nctexan]

Oh okay give that a shot. The sequence may be different if you have ABS, as pointed out by Barkz. Question is the pedal firm at first then creeps to the floor with the car running? If so you have a caliper issue.

I do have ABS. I will update with more detailed symptoms after lunch time, can't wait to drive it this is the first time I've driven it to work since I finished this evolution on the car!

 

[nctexan]

So I forgot a detail that I've experienced before I drove it yesterday which I am still seeing. When I depress the brake pedal, and then release, the engine RPM's rise and then fall again.

 

[Backlash2032]

So I forgot a detail that I've experienced before I drove it yesterday which I am still seeing. When I depress the brake pedal, and then release, the engine RPM's rise and then fall again.

Normal.

Did you bleed the brakes in this order? RL, RR, FL, FR?

 

[barkz]

^already been brought up