datsun79z Posted August 8, 2008 Share Posted August 8, 2008 Hello all; I've searched for a satisfactory answer to the following question but have not found the answer I need. Any help from fellow Z'sters will be appreciated. The problem: The car a 1980 ZX 2+2 and exhibits excessive braking force which is biased towards the front. Originally it was an auto, and is now a 5 speed manual. What I've done: I have installed two new master cylinders. Each time they were bench bled. The system has been succesfully bled three times. No change was noticed. I have ensured the brake line fittings are installed properley at the NP fitting. This is the fitting Nissan has mounted to the frame on the passanger side, under the hood. The check valve and brake booster have checked out according to the Factory Nissan manual. Vacume is being maintained per spec. I found no mention of a proporting valve or other related device in the Factory Nissan manual, nor have I located such a device on the car. Did I over look it? With the back of the car off the ground, out of gear, parking brake off and rotating the wheel/tire assembly by hand a minor amount of dragging noise can be heard. I believe this should indicate there is not excessive piston travel in the calipers. Minor pressure on the pedal locks both rear wheels up. There are no visible hydraulic leaks. DOT 3/4 is being used. All brake lines are of the stock variety. When braking, the car acts as though it has to much boost from the vacume booster. I did remove the booster and brake lines prior to painting the car. Could there be two differant boosters available (one for ZX's and another for 2+2's)? The front end brakes excessively hard. After coming to a full stop the brake pedal goes most of the way to the floor. Pumping of the brake pedal does little to bring it back up to where one expect it to be. With the engine off, I always have a firm pedal. Braap and Ron Tyler have looked at it as well as a Nissan mechanic with 30 plus years of experience. The three of them are baffled as am I. Input from fellow Hybrid Z members will be greatly appreciated. Regards; Michael Hintz Quote Link to comment Share on other sites More sharing options...
MJLamberson Posted August 8, 2008 Share Posted August 8, 2008 Do a search for "reaction disc" and see if what comes up helps, I BELIEVE it will. Quote Link to comment Share on other sites More sharing options...
Challenger Posted August 8, 2008 Share Posted August 8, 2008 Sorry cant help with the issue, just wanted to say that was the PERFECT post! Very well executed, so much better than all the illiterate posts Ive been seeing. Thank you!! Quote Link to comment Share on other sites More sharing options...
Hyuri Posted August 8, 2008 Share Posted August 8, 2008 Is the dragging coming from your brakes? If so, there's something wrong: disk brakes shouldn't have any contact when they aren't applied. Have you inspected for warpage and glazing? Other than "inspect everything", I can only suggest getting a FSM (http://xenons130.com/reference.html) and checking out the troubleshooting table. Quote Link to comment Share on other sites More sharing options...
blueovalz Posted August 8, 2008 Share Posted August 8, 2008 Oddest symptom I've ever heard. If indeed everything is correctly installed, then my guess is the booster is at fault. The vacuum is metered I believe, and if that metering no longer exists, or is faulty, then the full vaccum signal might be applied to a fairly large diagram (I believe the ZX diaphram is larger than the Z). I would disconnect the vaccum hose and drive it that way to observe the difference (be sure to block off the hose toward the engine or you'll have a massive vacuum leak). My brakes have a very slight drag as well. I've never had a set of disc brakes that did not "scrap" as you turned the wheel (jacked up and hand spun), but it must be very slight because it doesn't slow the wheel down. Remember, the seals around the pistons will retract the piston a slight amount, but (depending on the design), the pads are usually (not both of them anyway) attached to the piston, so once the piston retracts, the pad(s) may still rest against the rotor with no ill effect. Quote Link to comment Share on other sites More sharing options...
johnc Posted August 8, 2008 Share Posted August 8, 2008 +1 on the reaction disk (disc?). Quote Link to comment Share on other sites More sharing options...
datsun79z Posted August 9, 2008 Author Share Posted August 9, 2008 Gentlemen; In particular, Mr. MJLamberson, Mr. Challenger, Mr. Hyuri, Mr. Blueovalz and Mr. JohnC. Thank you for the advice and insight on this query. The Factory Nissan manual does not describe a 'Reaction Disc' within the braking system section. However I will continue to pursue the matter further. Would anyone care to describe what the 'reaction disc' looks like and to what side of the vacume booster it should be located on? Again, thank you all. If I can ever be of assistance please feel free to contact me directly at datsun79z@aol.com. Regards; Michael Hintz Quote Link to comment Share on other sites More sharing options...
MJLamberson Posted August 9, 2008 Share Posted August 9, 2008 I would use the hybridz search function, There are pics of it and everything. iirc its in the booster and falls out of place on occasion, its a simple matter of putting it back where it belongs. But dont trust me, search Quote Link to comment Share on other sites More sharing options...
woldson Posted August 9, 2008 Share Posted August 9, 2008 When braking, the car acts as though it has to much boost from the vacume booster. I did remove the booster and brake lines prior to painting the car. Could there be two differant boosters available (one for ZX's and another for 2+2's)? The front end brakes excessively hard. After coming to a full stop the brake pedal goes most of the way to the floor. Pumping of the brake pedal does little to bring it back up to where one expect it to be. With the engine off, I always have a firm pedal. I'm not sure I get this. If you have front bias (which is normal) when you bearly touch the pedal it is locking up the front?,,,,,,Yet you have to put the pedal to the floor? Then the pedal won't return, so the breaks are still engaged?? Quote Link to comment Share on other sites More sharing options...
johnc Posted August 9, 2008 Share Posted August 9, 2008 The Factory Nissan manual does not describe a 'Reaction Disc' within the braking system section. Yes it does. Page BR-21 has a diagram that shows he reaction disc (#18) and page BR-25 specifically mentions that you need to make sure the reaction disc doesn't fall off the push rob when adjusting push rod length BR-25. The reaction disc has a maintenance requirement that it gets replaced every two years as part of Repair Kit A per the FSM. EDIT: Oops. I quoted from the 240Z shop manual. Sorry. Quote Link to comment Share on other sites More sharing options...
JMortensen Posted August 9, 2008 Share Posted August 9, 2008 Yes it does. Page BR-21 has a diagram that shows he reaction disc (#18) and page BR-25 specifically mentions that you need to make sure the reaction disc doesn't fall off the push rob when adjusting push rod length BR-25. The reaction disc has a maintenance requirement that it gets replaced every two years as part of Repair Kit A per the FSM. Ha! Replaced every two years! As if... Quote Link to comment Share on other sites More sharing options...
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