Braking issue diagnosis

[JRusty15]

I'm having some trouble tracking down an issue with my braking system. Some backstory, it's a 91 240sx automatic with ABS, 100% stock. Picked it up with a soft brake pedal that needed to be pumped to get good braking force. Previous owner told me it probably has air in the lines. He said he changed the pads and replaced the calipers.

My first thought was a bad master cylinder, so I purchased a new one from Rock Auto. I bench bled the master, and installed it in the car. Then bled at each wheel. I went for a drive and the brake pedal was even worse. So I looked in the manual, and realized I needed to bleed the abs unit as well. So I bled the system again following the directions in the manual for the correct order. I bled each bleeder A LOT to make sure no air was coming out. I used a clear tube and verified no air bubbles in the tube at all. So I went for a drive again and still have a soft pedal. When the pedal hits the floor I feel some braking force.

So next step I checked the service manual for correct operation of the brake booster. I followed each step with successful results indicating the booster is operating correctly. Plus from what I've read, a bad booster would yield a hard brake pedal instead of a soft one.

Can anyone help me with some new diagnostic steps? I'm not sure what else could be the issue. Any help is appreciated.

[Wardin]

Which calipers did he replace? All 4, or just fronts or rear? The rear calipers on our cars are a twist back piston which some designs dont always re-adjust once installed just by pumping the pedal and need to be adjusted by taking the caliper off and twisting it back out to get the correct travel for the piston. Assuming you have checked for leaks and seized caliper slide pins and its bled properly that can give a very soft pedal.

[JRusty15]

I think he said he replaced all four, but I will have to check with him. Definitely something I can look into. Thanks!

[JRusty15]

I texted the previous owner and he said he replaced all four calipers. He said he had to rent the tool from the parts store to install the calipers but should be adjusted properly.

[DA KAOS]

If you can get hose clamps you can check if your rubber hoses are deteriorated. Or just visually inspect them. If they are soft or look old replace them. if you have the clamps pinch them off near the caliper to see if the pedal is still soft. If so you have bad lines. You can bleed w the abs fuse out, or try and get a few good abs stops then bleed the brakes. Sounds like air to me but I would see if the abs system gets bleed first.

Sent from my SPH-L710 using Tapatalk

[JRusty15]

Might be worth replacing the brake hoses anyway. Braided lines will be better for drifting anyway I'm sure.

Sent from my XT1080 using Tapatalk

[dorifto_demon]

got the same issue..
but with an honda ek..
new endless pads, new mc, new project u rotor, new braided hose all 4..
pedal was down to the floor..

replace the booster..
problem solved..

might wanna try that..

[JRusty15]

I don't understand how the brake booster can cause this issue?

Sent from my XT1080 using Tapatalk

[DJ 21o3]

weren't the automatic boosters stronger than the ones that came in the manual tranny cars? If so, shouldn't that make the pedal feel softer?

In the end, i would say replace the lines and then rebleed again. Made one hell of a difference in my Dad's DD but that was a chevy tracker... It had the same symptom with an extremely soft pedal and some braking force felt towards the bottom.

[PW180SX]

Having this same problem with mine, except here's my issue.

91 240SX with ABS

Rebuilt front calipers
New braided lines up front
brand new BMC, bench bled
Bled lines and ABS unit thoroghly, no air in the system at all

Pedal still sinks to the floor, but will build pressure if I pump it.

Could this be a BMC reservoir/cap issue? That's the only thing I didn't replace, not sure if the OP did either?

[JRusty15]

My bmc came with a new reservoir. Gonna try a vacuum bleeder this weekend and see what happens

Sent from my XT1080 using Tapatalk

[JRusty15]

I vacuum bled everything in sequence. I had a helper operate the brake pedal and made sure there were no leaks. Also used a pry bar to check that the wheels moved when brake pedal was up and didn't move when the pedal was down. When the car is off the pedal is firm but once the motor is running the pedal goes to the floor. When driving if I push the pedal to the floor its soft but the car slowly stops. If I pump it two or three times it feels normal. I even managed to get the abs to kick in once or twice. Also once or twice after a full stop the motor seemed to idle poorly. After I gave it gas it was fine. Any ideas?

Sent from my XT1080 using Tapatalk

[dorifto_demon]

I don't understand how the brake booster can cause this issue?

Sent from my XT1080 using Tapatalk

i was under the same impression that a problematic booster will cause the pedal to be rock hard..

the owner of the ek i did destroyed the brakes during circuit racing..

came to the shop, with destroyed front pads, front discs, and master cylinder (he was using a 15/16th honda mc)

replaced the front pads to an endless unit, front rotors to a project u, and a brand new 1" type r mc (which is 1/16th bigger than the previous setup)

vaccuum bled, and manual bled countless times. with the engine off (no vacuum assist) the pedal felt fine. but once the engine running, pedal went to the floor. bled it again with engine running. still the same

the i changed to a different booster, problem solved. nice solid pedal feel. no spongy, no nothing. it felt great.

i have yet to pinpoint the exact mechanism in the booster that caused it.

so yeah. give it a try. its just 6 nuts anyway. :)

[JRusty15]

Its not the work yo replace it, its the $140 to replace something that might be broken.

Sent from my XT1080 using Tapatalk

[blueshark123]

I have a booster u can have for 10 bucks off a s14 abs they work on s13's im 25 mins from rockaway text me 973 513 3613

[JRusty15]

Sweet! Text on the way

Sent from my XT1080 using Tapatalk

[Sileighty_85]

Make sure Bleeder Valves are on the top of the caliper.If they are on the bottom the air will not bleed out.

[JRusty15]

They are on top in the back. The fronts are basically in the middle.

Sent from my XT1080 using Tapatalk

[Sileighty_85]

Middle? Shouldnt be, should look like thishttp://www.sr20tuning.com/brakes02.jpg

[JRusty15]

Yea that's what it looks like. The bleeder valve is just facing towards the front bumper instead of facing up like the rears.

Sent from my XT1080 using Tapatalk

[godsmack]

If you pinch the vacuum line going to the booster and you get pedal then you have a bad booster. Yes in some cases a bad booster will just return the car to manual brakes, but in some cases and I'm not exactly sure on what goes internally, you will have the issue that you have now.

[JRusty15]

Ok thanks for the info. I will check that asap.

Sent from my XT1080 using Tapatalk

[Sileighty_85]

If your issue had anything to do with your brake booster it would be that little rod that pushes on the BMC piston. Some are adjustable on cars cannot remember if S-chassis are. Vac has nothing to do with it.When you bench bled the BMC was it on the car or off?Do you have any fluid loss after braking?

[JRusty15]

Bench bled the BMC off the car and no visible fluid loss after driving/braking

Sent from my XT1080 using Tapatalk

[JRusty15]

I bled the brakes again today starting with the master cylinder. With it still on the car I disconnected the brake lines and attached the adapter from the bleeding kit. From there hoses were attached with the other end in the reservoir. Turns out there was a good amount of air stuck in there still, especially on one of the ports. So I pumped the brakes until there were no more air bubbles in the lines. This took a while.

So I then went and bled the rest of the brakes again in sequence. This time I had my helper pump the brakes 3 times and hold the 3rd while I cracked open the bleeder, closed the bleeder and then my helper let the brake pedal up. There wasn't too much air in the brakes, but got it all out. Went to drive the car and initially the pedal felt pretty good. After like 60 seconds though, the issue remains.

I can push the pedal to the floor and some braking force is applied, but it's not 100%. If I pump the brakes, I get fell force. So I wanted to test and see if maybe the check valve going to the brake booster was bad. I tried blowing in it both ways and couldn't really tell much from that. So I took the valve out completely and blocked both ends of the vacuum line. With the car running the brake pedal feels significantly more firm with the vac lines blocked than with the check valve in place. This is leading me to think the valve is bad and the booster is just leaking out air back into the line. So I ordered a new valve and will try that once it comes in to see if that is in fact the issue.

[JRusty15]

I think these brakes will be the death of me. Or the cause for me to tear my hair out. I installed a new check valve for the brake booster today and bled the brakes AGAIN. I'm very confident there is no air left in the system due to how much I bled everything. Went for a drive again and the pedal still goes to the floor. However, I noticed that if I was going faster and he RPMs were up, the brake pedal felt firmer. Also, if I push the pedal slowly, the pedal is soft. If I slam my foot on the pedal, it feels harder. Again if I pump the brakes, I get good braking force. I even took out the fuse for the ABS to see if that made any difference and it did not. I checked for vacuum leaks between the intake manifold and the brake booster and could not find any. However, if the car is parked and idling and I slam the brake pedal repeatedly, the RPMs take a dive. The car doesn't stall, but idle gets a little rough. I don't know if that means anything?

I also checked to see if the BMC was leaking between the booster and the master cylinder. Visual inspection showed no fluid so I unbolted it from the booster. Nothing was leaking. I held the bmc up to the booster and had my helper push the pedal slowly. No leaks.

I'm totally lost here and need some advice on what to check next. This is driving me crazy.

Here are some pictures on how I bled the brakes. I had my helper pump the pedal three times and hold it down on the third time. I would crack the bleeder valve for like 1 second and close it then tell my helper to let up on the pedal. I did this until I had clean fluid coming out of the hose with no air bubbles. The other end of the hose was NOT submerged in the fluid in the bottle. I did each wheel like 10+ times each including the ABS module.
http://i264.photobucket.com/albums/ii166/jrusty15/IMG_20140308_115628916_zpsc2mijsms.jpg (http://s264.photobucket.com/user/jrusty15/media/IMG_20140308_115628916_zpsc2mijsms.jpg.html)
http://i264.photobucket.com/albums/ii166/jrusty15/IMG_20140308_115633694_zpspz2uixqs.jpg (http://s264.photobucket.com/user/jrusty15/media/IMG_20140308_115633694_zpspz2uixqs.jpg.html)
http://i264.photobucket.com/albums/ii166/jrusty15/IMG_20140308_115636547_zpssnuk9yx1.jpg (http://s264.photobucket.com/user/jrusty15/media/IMG_20140308_115636547_zpssnuk9yx1.jpg.html)
http://i264.photobucket.com/albums/ii166/jrusty15/IMG_20140308_115642188_zpsjo9diade.jpg (http://s264.photobucket.com/user/jrusty15/media/IMG_20140308_115642188_zpsjo9diade.jpg.html)

[Sileighty_85]

Dude your calipers are on the wrong sides

If you swapped calipers sides then the bleeder screw would be more to the top allowing all the trapped air to get out

Right now all the air is up here, since air rises to the top you are unable to bleed the air out with the bleeder screw on the bottom.

If you swap calipers so that the bleeder screw is where the red circle is the the air will be able to escape

http://i73.photobucket.com/albums/i235/Neil_85/Misc/caliper_zpsf48d9fb5.jpg

[JRusty15]

Sounds like there could be something to that. I'll try it out next time I get to work on it. Thanks

Sent from my XT1080 using Tapatalk

[JRusty15]

Sileight_85 I can't thank you enough! I switched the front calipers and it worked like a charm!! Thanks!!!!!!

Sent from my XT1080 using Tapatalk

[jkfdhklfchil]

The fronts are basically in the middle.

http://newfinancehelp.com/hu3b.jpghttp://interinsurances.com/huht.jpg

[Sileighty_85]

haha no prob man

[JRusty15]

The fronts are basically in the middle.

http://newfinancehelp.com/hu3b.jpghttp://interinsurances.com/huht.jpg

Yea, but with them on the wrong side the bleeder valve sits below the brake line. This prevents that air bubble from ever coming out.