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Gsax

280z brake proportioning valve issue

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Posted (edited)

Hello guys I'm having issues with my 76 turbo swap z. I upgraded my rear to disk brakes maxima/300zx and Toyota vented up front. I installed a zx brake master cylinder and wilwood proportioning valve. I kept the brake distribution block by the firewall. Easy enough or so I thought. Now when I get the car out on the road it will run good for 30 minutes or so until my brakes heat up, my brake pedal goes stiff and brake response takes a dump. Any input would be awesome. 

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Edited by Gsax

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You do not have a proportioning valve problem.  You have a MC push rod issue. The "distribution block" is called a brake switch and all it does is turn a light on if the front or rear brake circuit looses pressure. Leave it alone. You do not need an adjustable proportioning valve unless you will be racing and experimenting with brake pads. The brake set up you have is biased towards the front (70% F - 30% R). So reducing rear pressure will make your brakes perform worse.

Your brakes are locking up because the push rod that goes into the MC is adjusted too far out. It should be adjust so that it just about touches the bottom of the depression on the end of the piston. The rod is preventing the piston from fully retracting which is causing the piston to cover the fluid return ports for the tanks.  So each time you you step on the brakes you are filling the calipers, but the fluid can't flow back out.

Do the following:

 

  • Relieve the pressure by opening the MC bleeders. Keep the wrench in the car.
  • You don't have to remove the MC. Just remove the two nuts and pull the MC away from the booster and push it aside.
  • To adjust the push rod, hold the rod with needle nose pliers etc and use a small wrench to turn the adjustment nut inward a bit. NOTE: If you pull the push rod too far out you may drop the reaction disk into the bottom of the booster and your brakes will not work until you replace it. You will be sad.
  • The push rod on my MC is about 13 to 15 mm out as measured from the tip of the adjuster to the face of the spacer block.
  • Staying close to home drive the car and brake several times and drive around. Note: if the brakes are dragging (dragging = push rod too long) or if the pedal travel is too long (long pedal = push rod too short).
  • If the brakes lock up, use the wrench you used on the MC bleeders and relieve the fluid pressure.
  • Drive home and readjust the push rod one or two turns:   Turn in (shorten) if brakes drag/lock up or Turn out (lengthen) if brake pedal is too long.
  • Repeat until it is perfect. Write down the measurement of the rod each time you adjust it.  Save the final measurement.

Push Rod Adjustment.jpg

Edited by Miles
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