Challenger Posted July 22, 2009 Share Posted July 22, 2009 Well little background. The car is a 78 280z, last summer I disassembled the brakes and suspension for an overhaul. I put on 83 280zx rear calipers (bleeeder at the top) and SS 240sx lines which are the the right connections just longer than stock. I left the master cylinder empty so we removed the MC and found the seals bad (dried out over the last year), so I got a refurbished one today. Bench bleed it, hooked it up and then I pumped the brakes a little. Theres a little pressure on the first 2/3 of the push and then at the bottom it looses any pressure and hits the floor. I go to bleed the back, hook up the vacume hand pump and build pressure, crack the bleeder and I just get bubbles and a little bit of brake fluid. It never goes completely to fluid and the brake pedal never changes. Weary that theres not even fluid going to the back brakes I pull the line off of the caliper and then pump the brakes to see if fluid comes out. Nothing, maybe just a little dribbling. Im not sure what to do now. This doesnt seem like a reaction disk problem but who knows. Could it be the 280zx calipers? The brakes worked fine before I pulled them apart and I need to get this thing on the road and brakes are important. Thanks Quote Link to comment Share on other sites More sharing options...
Challenger Posted July 22, 2009 Author Share Posted July 22, 2009 Also I just started the car to see if the booster would do anything but now there is NO pressure on the pedal at all. Turn the car off and its back to how it was. Thanks Quote Link to comment Share on other sites More sharing options...
jt1 Posted July 22, 2009 Share Posted July 22, 2009 Either you haven't got the master bled completely, the reaction disc has moved, or you need to adjust the pushrod on the rebuilt master. Process of elimination. John Quote Link to comment Share on other sites More sharing options...
madkaw Posted July 22, 2009 Share Posted July 22, 2009 I second that. The M/C probably needs bleeding. Quote Link to comment Share on other sites More sharing options...
skunk1551 Posted July 22, 2009 Share Posted July 22, 2009 we had to replace calipers rotors and booster on top of all the cheap stuff to solve our problem, but check the booster first. Quote Link to comment Share on other sites More sharing options...
johnc Posted July 22, 2009 Share Posted July 22, 2009 Most likely the brake pressure switch has been tripped. The valve moves towards the end of the car with the lowest pressure and shuts off that circuit. You need to remove the nut on the end of the switch, recenter the valve, then gravity bleed the system so there is fluid to all four ends of the car before pushing on the pedal again. Quote Link to comment Share on other sites More sharing options...
Challenger Posted July 22, 2009 Author Share Posted July 22, 2009 we had to replace calipers rotors and booster on top of all the cheap stuff to solve our problem, but check the booster first. Well the rotors and calipers are new on the back, front calipers are new also. I second that. The M/C probably needs bleeding. I did bench bleed it before putting it on but maybe it got some air in there upon installation. Its worth bleeding it again. Most likely the brake pressure switch has been tripped. The valve moves towards the end of the car with the lowest pressure and shuts off that circuit. You need to remove the nut on the end of the switch, recenter the valve, then gravity bleed the system so there is fluid to all four ends of the car before pushing on the pedal again. I only found that its right under the master cylinder in the engine bay. Is this the right "switch"? So how do you gravity bleed it? Thanks for all the responses. Quote Link to comment Share on other sites More sharing options...
johnc Posted July 22, 2009 Share Posted July 22, 2009 Gravity bleeding means opening up all 4 brake bleeders and let gravity move the fluid out to the calipers. May take a little while. You can also use your vacuum pump. The mistake you made was pumping the brake pedal before making sure fluid was at all four calipers. And yes, the switch is under the MC. Quote Link to comment Share on other sites More sharing options...
Challenger Posted July 22, 2009 Author Share Posted July 22, 2009 Alright thanks. I just pulled off the MC and booster to rebleed the MC and check the reaction disk/clean the booster. I assume I gravity bleed it once the MC is back on the car. When you gravity bleed it you dont touch the pedal at all correct? Thanks Quote Link to comment Share on other sites More sharing options...
Challenger Posted July 22, 2009 Author Share Posted July 22, 2009 The booster is fine, the reaction disk was in place. I went ahead and cleaned it up and repainted and its mounted again. (looks a milllion times better) So Im still trying to figure out the brake switch part... as well as the gravity bleed. With the brake pedal in the stationary position wont the master cylinder not provide any fluid to drain by gravity into all the lines? Quote Link to comment Share on other sites More sharing options...
Administrators BRAAP Posted July 22, 2009 Administrators Share Posted July 22, 2009 (edited) Derek, I agree with John. Could very well be the pressure difference switch over-traveled/hyperextended, therefore blocking off fluid movement to the rear brake circuit. The pressure switch is one of those items, that in my opinion for any HybridZ is useless and serves no real function. If you stop to think about it for a minute, what is its purpose? Its purpose is to give the driver a visual indication on the dash, (illuminating the BRAKE light) that there is an improper pressure balance between the front and rear brakes, 99% of the time due to leak in either the front or rear brake circuit or the introduction of air into the system for some reason, such pads worn out, i.e. low brake fluid. This switch is wonderful for the secretary who has no car knowledge whatsoever, letting her know there is a problem with her brakes before it becomes a serious safety issue. Now being HybridZ guys, not only are we savvy with how and why the different systems of our cars function, we are also quite intimate with our cars. We know when something is even slightly amiss with our car. We can feel, hear, see, smell “any” change out of the norm, as such we are generally on top of these issues before they become real safety issues. Where am I going with this? Our brakes! We can FEEL when our brakes are having an issue such as air in the system, leak, etc, way before that pressure switch gives us that visual indication on the dash with the bright red “BRAKE" light illuminated! Derek, by your post here, you are obviously quite in touch with brake pedal feel, travel, etc, so I suggest to just delete that switch or remove the inner piston from it being sure to completely plug/seal off the center port where the piston used-to reside to keep the front and rear brake circuits from communicating with each other, start over with your bleeding. Most of the time, gravity bleed works quite well. Hope that helps, Paul Edited July 22, 2009 by BRAAP Typo's Quote Link to comment Share on other sites More sharing options...
Challenger Posted July 22, 2009 Author Share Posted July 22, 2009 Alright Ive been looking at some other pictures of s30's and see that I dont have the pressure switch. The pictures Ive seen along with my chiltons show a large metal body that looks similar to the prop valve. Mine is nothing like it and looks like this. It appears that it was already just T'd off. Quote Link to comment Share on other sites More sharing options...
Administrators BRAAP Posted July 23, 2009 Administrators Share Posted July 23, 2009 (edited) Derek, If memory serves, the '78 did away with the switch which also acts as the "T" for the front brakes, (splits the front circuit into Frt-left and Frt-right), going to reservoir caps with "floats" that indicate with the same "BRAKE" light, low brake fluid! Then used and actual "T" as you pictured for the split to left and right front brakes. Sorry about the confusion. Here is the switch we are reffering to which your car most likely does not have. This is a '75 car; With that, I'm more inclined to think either you didn't get the Master bench bled correctly there fore no fluid moving to the rear or the master cylinder may have defect internally, i.e. seal in backwards, torn, etc. Your description of how the brake pedal moves/feels is weird. Also, lets try to keep the booster discussions out of this for now. One problem at a time. The brakes will still function 100% without the brake booster, (what you will notice is elevated pedal pressures, i.e. travel and feel will still be the same!) so just disconnect the vacuum line and be sure to remove the one-way check valve that is in that vacuum line as well. With the engine off, the pedal should still not go to the floor and the reaction disk/booster would not cause that even if either was having a problem, especially if the engine is not running and booster is disconnected, check valve removed, i.e. no vacuum present. In short, deal with only one issue at a time, once that is resolved, then deal with a possible booster issue. If the Master cylinder is an issue, there is no way you can effectively diagnose a booster issue as the pedal travel and feel most likely wont be right any way! Again, ignore the booster till the master cylinder is sorted out. Edited July 23, 2009 by BRAAP Quote Link to comment Share on other sites More sharing options...
Challenger Posted July 23, 2009 Author Share Posted July 23, 2009 Alright. Anyways, I put it all back together so I guess now is the time to gravity bleed it. Should I just attach hose to all the bleeders (open them) and then let it sit? Do I need to put the pedal to the floor and than gravity bleed it? I guess whats the best way to make sure all the lines are full of fluid. Quote Link to comment Share on other sites More sharing options...
MAG58 Posted July 23, 2009 Share Posted July 23, 2009 No no, don't touch the pedal until after you gravity bleed it! If you do you'll just end up in the same place which you started. Gravity bleed it (or assist with your vacuum pump) until you see fluid coming out of all four bleeders before. Once you get fluid to all the cylinders you can start playing with the pedal again. Quote Link to comment Share on other sites More sharing options...
Trevor Posted July 23, 2009 Share Posted July 23, 2009 [qote]The pressure switch is one of those items, that in my opinion for any HybridZ is useless and serves no real function. If you stop to think about it for a minute, what is its purpose? Its purpose is to give the driver a visual indication on the dash, (illuminating the BRAKE light) that there is an improper pressure balance between the front and rear brakes, 99% of the time due to leak in either the front or rear brake circuit or the introduction of air into the system for some reason, such pads worn out, i.e. low brake fluid. This switch is wonderful for the secretary who has no car knowledge whatsoever, letting her know there is a problem with her brakes before it becomes a serious safety issue. Most likely the brake pressure switch has been tripped. The valve moves towards the end of the car with the lowest pressure and shuts off that circuit. You need to remove the nut on the end of the switch, recenter the valve, then gravity bleed the system so there is fluid to all four ends of the car before pushing on the pedal again. Based on the description of fluid "dribbling" out of the open line when the master cyl was applied, I agree with John C, the switch needs to be re-centered. Not electrically - actually hydraulically. When there is a big enough differential in back pressure between the 2 halves of the system the switch closes a mechanical valve that shuts off pressure to the open half of the brake system (front or rear - in your case rear) to prevent total fluid pressure loss and total brake failure. Once "tripped" the switch serves as a failsafe preserving half of the divided system. I have fought this on several cars when re-filling and bleeding a dry brake system. Quote Link to comment Share on other sites More sharing options...
Challenger Posted August 3, 2009 Author Share Posted August 3, 2009 Alright, I let it gravity bleed for awhile and sure enough the mc slowly drained and I fillled it a couple times. Today we tried to bleed it and it seemed to put through clean fluid but the pedal is soft. I have enough brakes to keep me from going down the driveway and thats about it. Im also finding when I push on the pedal quickly I here a psst sound from the MC/booster area. Not sure what it is. Quote Link to comment Share on other sites More sharing options...
Challenger Posted August 3, 2009 Author Share Posted August 3, 2009 Ok, just noticed something. Ive got the large resevoir on the front of the MC with the small one closest to the firewall. I looked at some pictures of 78s and it appears I have it backwards. Would this make the different? Also is it a possibility that since my MC is meant for drums in the back would I need to switch to the 280zx MC? Quote Link to comment Share on other sites More sharing options...
jt1 Posted August 4, 2009 Share Posted August 4, 2009 Usually the larger reservoir is for the front brakes, but I don't think that would cause your problem. A slight hiss from the booster is normal. Did you ever let the master drain all the way? If so, you've probably still got some air in the system. Sometimes when you completely drain a system it takes a lot of bleeding to get all the air out, like running a couple of quarts of fluid thru the system. You also need to bleed all four corners, not just the rears. Since the ZX is a larger bore master, it will give you a firmer pedal, after you get all the air out. jt Quote Link to comment Share on other sites More sharing options...
Challenger Posted August 4, 2009 Author Share Posted August 4, 2009 I did bleed all four. I guess I can as you say just bleed it for a couple days strait to get all the air out. Hopefully its just air. Thanks Quote Link to comment Share on other sites More sharing options...
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