MikerZ Posted June 28, 2015 Share Posted June 28, 2015 So I'm stuck in the bleeding nightmare... Yesterday I replaced the front brake lines because they were in huge need of replacement. I then swapped the rotors while I was at it and the pads as well. All this was done to the front of the vehicle. Now as I got ready to bleed the system, I was able to bleed the rear breaks, but when I got to the fronts, I don't get any pedal pressure at all... Could this be due to a faulty brake booster or MC? Any help would be greatly appreciated! Thanks in advance Quote Link to comment Share on other sites More sharing options...
NewZed Posted June 28, 2015 Share Posted June 28, 2015 MC's have been known to destroy their seals when they see a new part of the bore. The booster just amplifies the force from your leg, Brake fluid is denser than air. There must be some pedal pressure, there's a spring and you're also pushing on the rear brake system. Quote Link to comment Share on other sites More sharing options...
74_5.0L_Z Posted June 28, 2015 Share Posted June 28, 2015 You do have the bleeders on top (right)? If you swap the caliper from left to right, the bleeder will end up on the bottom and you will not be able to bleed the air. Quote Link to comment Share on other sites More sharing options...
MikerZ Posted June 28, 2015 Author Share Posted June 28, 2015 Bleeders are on top and yes I definitely suspect that is in fact the MC. In going to replace it with one of the reman ones for the mean time... This would have to be bench bled and then finish off with the proper bleeding pattern/procedure. Thanks guys! Quote Link to comment Share on other sites More sharing options...
johnc Posted June 28, 2015 Share Posted June 28, 2015 Is the brake pressure switch tripped? If you did not gravity bleed the fronts first the switch has probably tripped cutting of the front circuit. Quote Link to comment Share on other sites More sharing options...
Miles Posted June 29, 2015 Share Posted June 29, 2015 (edited) The most likely scenario is that you allowed fluid to drain out of the MC tank for the front brakes. This will introduce air into the front brake lines making it impossible to bleed the front calipers. You will have to bench bleed the MC if this happened. This would be a good time to just replace it. The other scenario is that the MC was failing before the brake job and now the front part of the MC has failed. It has happened to me when I replaced the 240SX calipers on my 240Z. When I road tested the car after the brake job the MC let go. It happens. Now the infamous brake switch. All it does when the pressure drops in the front or rear brake circuit is that a spring loaded shuttle moves to the low pressure side blocking fluid flow to the"leak" and turning a red light on in the speedometer. That is it. The shuttle is spring loaded at each end and is self centering once you let off the brake pedal. There is no mechanism inside the switch to lock it one way or the other. Study the picture below. It is just a hydraulically operated light switch that moves when there is a pressure difference between the front and rear brake circuits. Hence "differential pressure safety switch". I replaced mine with a three way brass line connector years ago because I don't need a light to tell me that my brakes are failing. Do not gut the switch either as you will then tie the front and rear brakes together. There are no repair parts because Nissan built it as an expendable item in case the switch failed. So you either have air in the front circuit and or the MC has failed. There is nothing wrong with the brake booster unless you took the MC out and dropped the reaction disk inside the booster. That has a whole different set of symptoms. My 72 240Z had the switch as in the second picture. The second picture is from the FSM. Edited June 29, 2015 by Miles Quote Link to comment Share on other sites More sharing options...
NewZed Posted June 29, 2015 Share Posted June 29, 2015 Nissan changed the switch design sometime in the first few years of the 240Z. You're showing the newer design. Quote Link to comment Share on other sites More sharing options...
Miles Posted June 29, 2015 Share Posted June 29, 2015 (edited) Newzed My 72 240Z had the switch like the second picture (picture from FSM). Is the first picture the early or late model switch? With either switch the springs at each end recenter the piston. After reading many conflicting stories about the switch I removed mine and dissected it to see how it worked. Manually moving the piston against the spring and and then releasing it the piston returned to the center resting position. In the later Z cars (280ZX) the switch was replaced with level sensing master cylinder caps Edited June 29, 2015 by Miles Quote Link to comment Share on other sites More sharing options...
NewZed Posted June 29, 2015 Share Posted June 29, 2015 Here's a picture of the older switch. I really don't know how the older ones work, I've just seen that they need to be reset if they get tripped. Maybe they have a tendency to get stuck? My 76 had the switch and it fixed itself as soon as I fixed the problem (calipers on wrong sides). The OP never said what year car he's working with. Quote Link to comment Share on other sites More sharing options...
Miles Posted June 29, 2015 Share Posted June 29, 2015 (edited) The FSM just says to replace the switch if it is defective. There is no latching device inside the switch, as in my 72 240Z switch , which would capture the piston and hold it. The springs at each end insure that the piston re-centers itself. I suppose that corrosion build up could cause the piston to freeze in the bore in which case the switch would be thrown away and replaced as per the FSM. Note if you take the switch apart and you tear a seal there are no new seals available from Nissan. You can bench test a switch by placing it in vise, attaching a volt meter to the electrical contact, and then push the piston in and out. The volt meter will turn on and off as the piston moves. Is the resetting of the pressure differential switch a Z car myth? Ancient Alien theorists would say yes. As per the guy with the boner hair on the History Channel. Edited June 29, 2015 by Miles Quote Link to comment Share on other sites More sharing options...
johnc Posted June 29, 2015 Share Posted June 29, 2015 No its not a myth. On my 1970 and my 1971 and three customer cars that were brought into my shop the switch would trip and stay locked until you filled the dry circuit and pumped the brake pedal. Then you would hear a click when the switch reset. Whether by design or age/malfunction, that's how they work. Quote Link to comment Share on other sites More sharing options...
Miles Posted June 29, 2015 Share Posted June 29, 2015 John Also true for 72 and newer? Quote Link to comment Share on other sites More sharing options...
Miles Posted June 29, 2015 Share Posted June 29, 2015 (edited) If for any reason that the differential pressure safety switch is suspected of being "tripped" place one lead of a volt meter or test light on the electrical contact of the switch and the other lead on the body of the car. If you get a reading or a light then the switch is tripped. The switch is not repairable. You can replace it with a used switch or replace the switch with a three way connector for the front brakes and connect the rear brake line directly to the master cylinder. Edited June 29, 2015 by Miles Quote Link to comment Share on other sites More sharing options...
MikerZ Posted June 29, 2015 Author Share Posted June 29, 2015 Ok, so I was able to get the brakes going. As suspected, the MC appeared to be bad, but found out that the lines and Oem(stock 78) MC were clogged with turd... I had to pressure blow the crap out of them and the mc I just replaced it with a reman from oreillys. Thanks for the support guys, I greatly appreciate it! Quote Link to comment Share on other sites More sharing options...
johnc Posted June 30, 2015 Share Posted June 30, 2015 I don't know if the '72 or later switches "stick." I removed them from all my cars when I did any brake upgrades. Quote Link to comment Share on other sites More sharing options...
JSM Posted July 7, 2015 Share Posted July 7, 2015 In the later Z cars (280ZX) the switch was replaced with level sensing master cylinder caps Curious, so the switch is just a safety device so if fluid is lost either in the front or rear reservoir the brakes will still work? Quote Link to comment Share on other sites More sharing options...
Miles Posted July 7, 2015 Share Posted July 7, 2015 (edited) JSM See pictures above. If a brake line or caliper seal fails, then the piston shuttles to the low pressure side of the switch and blocks fluid from flowing on that part of the brake system. So, for example, if a front brake line broke the piston would move to block fluid going to the front brakes. Theoretically, you would still have rear brakes. I replaced mine by connecting the rear brake line directly to the master cylinder and installing a three way M10 x 1.0 inverted flare connecting block where the switch was bolted to the fender to connect the front lines to the master cylinder. After 40 years I wouldn't trust the switch to work. Edited July 8, 2015 by Miles Quote Link to comment Share on other sites More sharing options...
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