Sketch_hs Posted February 7, 2010 Share Posted February 7, 2010 I just swapped for rear disk on my 1975 280z I keep reading about some valve or restrictor for the drums that I should remove? Feels like it need more pressure going to the rear brakes if anything, fronts lock up easily (at least in the rain!) Anything else that should be done in the system? I'll be upgrading the fronts next Thank you! Quote Link to comment Share on other sites More sharing options...
Xnke Posted February 7, 2010 Share Posted February 7, 2010 The "pressure regulating valve" is the brake proportioning valve, and you should either upgrade it to an adjustable unit, or leave it alone. What brakes are you running on the rear now? What brakes are you running on the front now? What brakes are you upgrading to on the front? Have you driven the car with the new rear brakes on, stock front brakes on, and stock proportioning valve? If you are running the 240SX rear calipers, 300ZX rear rotors, and the stock S30 front calipers, and stock S30 front rotors, then you should be about the same front/back ratio and should not need to change the proportioning valve. BUT you should be using the ZX 15/16" master cylinder, not just because it pushes more volume, but because the stock master is valved for front disk/rear drum brakes, and the ZX master is valved for front and rear disks. The different brake systems need different valve characteristics! IF these are the 240SX rear calipers, I would run the stock S30 calipers for a street car, and not go any bigger than the S12+8 calipers. I still haven't gotten to drive my car, though, so take my caliper reccomendations with a grain of salt. I've just been absorbing this from reading the brake FAQ, the brake Stickies, and reading posts from those who are running these brake setups both on the track, and on the street. Quote Link to comment Share on other sites More sharing options...
Sketch_hs Posted February 7, 2010 Author Share Posted February 7, 2010 I know what a prop valve is and does, but I was reading that there is a valve that maintains some pressure in the rear system for the drums. And I am experiencing a small amount of rear disk "dragging" after a stop. Quote Link to comment Share on other sites More sharing options...
Apex944 Posted February 8, 2010 Share Posted February 8, 2010 I don't know of any extra valve in the rear of a Z that holds the left over pressure in the back but I know that in the master cylinder there should be a check valve that holds residual pressure of 6 - 25 PSI to keep the seals on the wheel cylinder's from leaking brake fluid when they collapse from a lack of pressure. I just looked in the FSM for my '73 and it doesn't have any extra part's other than the 3 way connector in the brake line system so I'm assuming that in the Datsun Z car's this valve is in the master cylinder like most vehicles have that have rear drum brakes. Now there is an NPR valve that controls the front to rear pressure to keep the rear brakes from locking before the front's so it's basically your proportioning valve from the factory. It has other function's as well but not really worth posting in here when it can be more easily understood by reading about it in the FSM. My guess is that if you switch to the 15/16 master cylinder it shouldn't have the residual pressure valve like the early Z car's had because the ZX, as far as I know, had rear disc brakes from the factory so there was no need for a residual pressure valve since they have pistons instead of wheel cylinders. Quote Link to comment Share on other sites More sharing options...
Xnke Posted February 8, 2010 Share Posted February 8, 2010 Apex is exactly right; on the residual pressure valve being incorperated into the master cylinder on the 70-78 z's. You need a ZX master. Quote Link to comment Share on other sites More sharing options...
Sketch_hs Posted February 8, 2010 Author Share Posted February 8, 2010 Apex is exactly right; on the residual pressure valve being incorperated into the master cylinder on the 70-78 z's. You need a ZX master. Awesome. Is that a direct swap? Any booster upgrade I should consider? Maybe mine is just aged, but I feel like the pedal is too hard. Quote Link to comment Share on other sites More sharing options...
johnc Posted February 8, 2010 Share Posted February 8, 2010 Apex is exactly right; on the residual pressure valve being incorperated into the master cylinder on the 70-78 z's. You need a ZX master. No such thing. There is nothing on the stock S30 brake system that retains hydraulic pressure after the brakes are released. If your rear disks are dragging either the master vac pushrod is not allowing the MC pistons to fully retract or your e-brake setup on the rear disks is not properly adjusted. Quote Link to comment Share on other sites More sharing options...
Apex944 Posted February 8, 2010 Share Posted February 8, 2010 No such thing. There is nothing on the stock S30 brake system that retains hydraulic pressure after the brakes are released. If your rear disks are dragging either the master vac pushrod is not allowing the MC pistons to fully retract or your e-brake setup on the rear disks is not properly adjusted. Then what keeps the rear wheel cylinder's from leaking brake fluid when they collapse from the lack of brake pressure? Quote Link to comment Share on other sites More sharing options...
johnc Posted February 8, 2010 Share Posted February 8, 2010 Then what keeps the rear wheel cylinder's from leaking brake fluid when they collapse from the lack of brake pressure? Wheel cylinders collapsing? Quote Link to comment Share on other sites More sharing options...
johnc Posted February 8, 2010 Share Posted February 8, 2010 FYI... there is a brake pressure differential switch under the MC or on the firewall on S30s. It has a shuttle valve that will kill brake pressure to the front or rear circuits if the pressure difference is greater then 185 to 242 psi. This shuttle valve retracts after the brake pedal is released. Quote Link to comment Share on other sites More sharing options...
JMortensen Posted February 8, 2010 Share Posted February 8, 2010 Wheel cylinders collapsing? He means the seals in the wheel cylinders. Kind of an urban legend that I heard growing up that most/all drum setups use a valve incorporated in the master because if they didn't the seal would not have any mechanism to hold it's pressure against the wheel cylinder bores. As you drive vibrations would allow the fluid to slip past the wheel cylinder seal cups, creating a leak. With just a small amount of pressure you can force the seals against the inside of the cylinder and prevent such a leak. I have not verified this claim, nor have I seen anyone dig the part out of a disc/drum master to show it, but I did believe it existed for a long time. Now I'm not so sure. If you look up residual pressure valves (ala Wilwood or similar), they generally are sold as a cheater part that "fakes" a proper drum adjustment, and also as a device that prevents the fluid from flowing back out of the master if the master is mounted below the calipers or drums. Might be something that ITS guys should be looking at... http://www.wilwood.com/products/006-MasterCylinders/010-RPV/index.asp http://www.sachserodshop.com/pdf/55.pdf Quote Link to comment Share on other sites More sharing options...
johnc Posted February 8, 2010 Share Posted February 8, 2010 OK, I've never heard of that urban legend. There isn't anything in the S30 braking system that retains brake line pressure. Inside the rear wheel cylinder there's a small spring that keeps the piston in contact with the shoe and the shoes are retracted by the various installation springs. Regarding the residual pressure valves, I know its been tried by some ITS guys before I started racing but I don't know the results. In the community I raced with no one was running that kind of valve. We just adjusted the rear brakes manually before each session. Quote Link to comment Share on other sites More sharing options...
zcarnut Posted February 8, 2010 Share Posted February 8, 2010 John is correct…. The S30 Z car brake systems do not use a residual check valve to maintain a minimum pressure in the rear brake drum circuit. Rather, the rear drum brake wheel cylinder lip seals are spring loaded against the cylinder walls to prevent leaks. Yes, there are check valves in the Z car master cylinder, but these are very low pressure (1 to 2psi) valves that just prevent the brake fluid from being siphoned out the master cylinder reservoirs in the event that the master cylinder is at, or below, the height of the calipers (which could occur when you are parked on an incline). These anti-siphoning valves are used in both the front and the rear brake fluid circuits. Quote Link to comment Share on other sites More sharing options...
Sketch_hs Posted February 10, 2010 Author Share Posted February 10, 2010 Interesting stuff guys So is there still a valving benefit from swapping to the 15/16th ZX MC? I can get one pretty cheap (without the reservoirs ... do they swap over from the 75 z master??) Quote Link to comment Share on other sites More sharing options...
Sketch_hs Posted February 11, 2010 Author Share Posted February 11, 2010 anyone know? Quote Link to comment Share on other sites More sharing options...
zcarnut Posted February 11, 2010 Share Posted February 11, 2010 So is there still a valving benefit from swapping to the 15/16th ZX MC? I can get one pretty cheap (without the reservoirs ... do they swap over from the 75 z master??) No “valving†benefit, just a firmer brake pedal, especially with disc brakes. Only the 1979-81 ZX master will fit your booster and your reservoirs will likewise only fit on the 1979-81 ZX master. However, it is not recommended that you use the smaller size reservoir designed for the drum brakes when you change over to disc brakes due to the larger fluid requirement of the calipers compared to the drum cylinders. Quote Link to comment Share on other sites More sharing options...
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