Help with why I keep getting sinking pedal

[tery]

Hi,
So I replaced all the lines on my 90 SHO, Silvia, I know I got air in the lines and I've been trying one man bleeding with the bottle hose set up and seem to have gotten the air out, but I keep getting sinking pedal, after pumping for a while it'll get hard and stop, but then when I turn on the engine again it sinks to the floor. So then I pump and it comes up and eventually gets to where it is hard, but then if I rev the engine, it goes back to sinking again..
Am I looking at Master cylinder problems or leaking brake booster or ...what else?? Please send me any thoughts or previous similar experiences before I just have to take her in to "them"..actually, my shop here is pretty good. They'll even let me bring in my own parts and put them on for me, if its something out of my physical reach or ability

 

[SHOdded]

If you have ABS, the head unit could have air trapped in it. IIRC, braking hard once up to speed on a gravel road and repeating is one recommended way to fix the issue without needing to get a shop to do it.

But I would look at the master cylinder first, if all the lines look ok. If it's bad enough, the master cylinder can leak into the brake booster and ruin it, so be prepared for that also.

I have had a case where it was dirty brake fluid causing a sinking pedal, but since you just put in new fluid ...

 

[tery]

Thanks Manu,
Hey...tell me please, what would I see if the fluid is leaking back into the brake booster unit??

And, I think I have heard that ting about hard braking...Oh yeah, I saw it once on Eric the Care guy...Can't I just...not too crazily...smash-ish the brake pedal in the driveway??

 

[Mikey]

I had a strange time bleeding brakes after changing the lines. Same symptoms...bleed the brakes thoroughly...still had to pump 1/2 to attain full brake on all four discs.

Anyway (some body is gonna laugh at me for this) try bleeding the brakes with the car running. I may have just needed one more round of bleeding, but it fixed it for me.

Also start at the caliper farthest from the brake fluid reservoir, heard it helps too.

 

[tery]

hey, thanks...I'm not laughing but it makes me wanna ask if you did it by yourself with like a one man bleeder kit, or did you have someone putting on pressure while you open/close? I'm asking because there's no question that with the engine running, the pressures and everything is all different, so if I'm bleeding them but its not activating the ABS motor cause that is electric...then I'm maybe not getting the air out of the ABS unit...
Thanks for the reply..

***Fred??

 

[SHOdded]

Thanks Manu,
Hey...tell me please, what would I see if the fluid is leaking back into the brake booster unit??

I haven't had personal experience with BOTH failing (just the BB), so I can't say for sure. Check the seal(s) between the MC and the BB? I suppose you could bench bleed your old MC once you take it off and prove a seal leak that way?

If the brake fluid level drops too quickly to correlate with pad wear, and there are no leaks in the lines or the brake hoses or calipers etc. then very likely it is an MC-related issue. My 95 did have a pinhole leak in the line running to the RR and it was very difficult to find.

Apparently it can go the other way as well
http://my.cardone.com/techdocs/PT 50-0001.pdf
http://my.cardone.com/techdocs/PT 10-0005.pdf

 

[rubydist]

no you cannot just mash on the pedal in the driveway. the gravel road thing works because you brake hard enough to slide the tires which forces the abs pump to do its magic which moves the air out of the abs unit either into the lines where you can bleed it out, or back up to the master cylinder where gravity will bleed it out.

the trick is not just any gravel road, but one with lots of loose gravel so its easy to get the tires to slide.

 

[tery]

Thanks much guys, I'll continue my quest and report back when I find the problem...
maybe I'll wait til the roads ice up here and try it on ice sliding sideways through an intersection with the ABS doing its thing...People will say.."look at that idiot,trying to get the air out of his ABS motor...shoulda done it on a gravel road"...

 

[luigisho]

or you can pay someone with the correct electronic bleeder to activate the abs and bleed it in a garage

 

[sperold]

The last picture in post #1 is what you are looking for.
http://www.shoforum.com/index.php?threads/lots-of-gen-1-and-2-parts-17x7-5-ar-wheels.118620/

 

[Mikey]

hey, thanks...I'm not laughing but it makes me wanna ask if you did it by yourself with like a one man bleeder kit, or did you have someone putting on pressure while you open/close? I'm asking because there's no question that with the engine running, the pressures and everything is all different, so if I'm bleeding them but its not activating the ABS motor cause that is electric...then I'm maybe not getting the air out of the ABS unit...
Thanks for the reply..

***Fred??

It was a two man job. Did three cycles on each caliper. Had my buddy pump the brakes for me and hold the pedal down while I worked the bleeder.

Thought for sure I was gonna get busted for that suggestion, hope it can help. Good luck !

 

[rubydist]

Thanks much guys, I'll continue my quest and report back when I find the problem...
maybe I'll wait til the roads ice up here and try it on ice sliding sideways through an intersection with the ABS doing its thing...People will say.."look at that idiot,trying to get the air out of his ABS motor...shoulda done it on a gravel road"...

LOL

its actually harder to do this on ice. the thing is that if you mash on the brakes too hard, and lock up all 4 wheels hard at the same time, the abs thinks you stopped real quick and does nothing. obviously, that is not real hard to have happen on ice, so the trick is to brake hard enough to get at least one wheel to skid, but not all of them at exactly the same time.

 

[Brian Mullins]

LOL

its actually harder to do this on ice. the thing is that if you mash on the brakes too hard, and lock up all 4 wheels hard at the same time, the abs thinks you stopped real quick and does nothing. obviously, that is not real hard to have happen on ice, so the trick is to brake hard enough to get at least one wheel to skid, but not all of them at exactly the same time.

Hi,
So I replaced all the lines on my 90 SHO, Silvia, I know I got air in the lines and I've been trying one man bleeding with the bottle hose set up and seem to have gotten the air out, but I keep getting sinking pedal, after pumping for a while it'll get hard and stop, but then when I turn on the engine again it sinks to the floor. So then I pump and it comes up and eventually gets to where it is hard, but then if I rev the engine, it goes back to sinking again..
Am I looking at Master cylinder problems or leaking brake booster or ...what else?? Please send me any thoughts or previous similar experiences before I just have to take her in to "them"..actually, my shop here is pretty good. They'll even let me bring in my own parts and put them on for me, if its something out of my physical reach or ability

I know this is old thread.. But maybe this can help someone in the future. My 94 atx brake pedal was going to the floor after chsnging calipers. One of the new calipers was messed up.. Piston would go in the opposite direction it was suppossed to go when applying the brakes.. Got another caliper, and everything worked great

 

[Devin]

How'd the piston go the opposite direction?

 

[Brian Mullins]

How'd the piston go the opposite direction?

Great question.. My mechanic & I have never seen that before... Just a bad part

 

[tery]

the piston can't go in the opposite direction from hydraulic fluid pumped into the line and into the caliper..
I did have a thought though about what I'm working on with it...It seems now to me that the front is cleared of air and when I do the sliding on gravel trick, it feels like mostly the fronts are activating and also by the look of the rear disks..less contact and wear...so I had the thought that I should try going in reverse and doing the same tactic to get the rear brakes to activate moorland lock them up to get ABS action... since in reverse they would get a lot more of the stopping force/reaction..I'd love to hear others thoughts on the matter.
The first two times I did it, things got a lot better and like I say, the fronts are activating from the pump I think and it also feels like the fronts start grabbing before the rear and that's when it takes a lot of pedal to get all four wheels stopping...thoughts??

 

[rubydist]

does that compensation valve in the rear lines work right on that car? most Taurii will not get a lot of rear brake action because most of them have that valve stuck in the unloaded position so not much pressure gets through to the rear calipers.

 

[tery]

Hey...Yes, I'm pretty sure because I had the whole thing apart and cleaned it etc and fluid does come out of both calipers easily when you crack the bleeder screw
What do you think about my backing up at 15 mph and hitting them?? Does it make sense that then the rears will get more action because the back of the car will dip and make the "gizmo" on the compensation valve/distribution block?.. engage more--since it is attached at that crossbar??

 

[rubydist]

possibly, I never tried that.

 

[tery]

Hey...well I tried it and now it appears and feels like the rr is air free, just the left rear is still not fully engaging the pad on the rotor, I'll try it some more and try to get the lr to "skid" as it were..and the pedal is better, I feel safe to drive it now, just don't like the feel of the pedal yet, thanks for all your help