Brake Issue...baffled

[dreco]

I have modern motorport rear disc brakes with AZC front brakes.  I have an adjustable proportion valve in the rear of the car.  15/16 master cylinder with stock brake booster.  Brakes were fine for a while.  Now a bad brake drag has developed in the front.    I changed master cylinders, rechecked booster shaft clearance and brakes still drag (after one pump of the brakes).  Both front calipers lock up.   I can bleed on either side and it frees up both sides.  Cant figure out where the residual pressure is coming from. Any thoughts?   Could it be the stock proportion valve in the engine bay? or could the rear proportion valve be the cause ( I wouldn't think this due to being on the rear circuit.)

Edited December 11, 2015 by dreco

[NewZed]

The rod from the pedal to the booster, in the cabin, might be too long.  Not allowing fluid back in to reservoir.  It's been discussed many times if you want to search around.  "Sticking brakes", "brakes won't release", etc.

 

If this was the case, pulling the clevis pin on the brake pedal should release the pads without opening the bleed valves.  If it's the booster, then loosening the MC from the booster should do the same.  A quick check of the mechanical pieces.

[dreco]

The rod from the pedal to the booster, in the cabin, might be too long.  Not allowing fluid back in to reservoir.  It's been discussed many times if you want to search around.  "Sticking brakes", "brakes won't release", etc.

 

If this was the case, pulling the clevis pin on the brake pedal should release the pads without opening the bleed valves.  If it's the booster, then loosening the MC from the booster should do the same.  A quick check of the mechanical pieces.

its not that.. checked that.  

[Apollyon12]

Some master cylinders have a valve in the rear circuit that keeps pressure in the system.   This is to keep a slight drag in drum systems.   Not sure if any of ours would have this.   My thought was that you have the front and rear swapped on the master cylinder.   When I replaced mine with a 15/16ths one, I had to.

Edited December 11, 2015 by Apollyon12

[NewZed]

its not that.. checked that.  

When the rod in the MC is back to its proper starting position it's impossible to hold any pressure in the hydraulic system.  The formerly pressurized system is completely open to the atmosphere.  Unless the cap on the reservoir is sealed and has all vent holes blocked.  It's just the way they're designed.  You can see it in the drawings.  

 

Unless, apparently, you have one of the special ones described above.

[NewZed]

Typical MC at rest.  1980 280ZX Tokico brand.

[Miles]

1. Tell us what year Z you have as there are differences in the hydraulic systems.

 

2. There are residual pressure valves in the outlets of the master cylinder. They look like a piece of rubber with a spring. Remove them.

 

3. You should not have two proportioning valves (PV).  Having two PVs can cause problems other than what you have mentioned here. If you have for example,  a 72 240Z, and you have installed a PV in the rear then the stock PV should be removed. On later model Z cars you need to understand what the PV does before removing or modifying it.

 

4. The other question is why do you need a PV in the rear? 240SX rear calipers are  typically undersized relative to most so called "front brake upgrades". So unless you  are experimenting with high friction rear pads to fix front - rear balance issues a rear PV is not required with 240SX rear calipers.

 

5. You can relieve pressure to the front or rear brake circuits by opening the respective bleeder on the MC if you need to limp home with dragging brakes.

 

6. In my experience, the front brakes will lock up after a few stops if the booster push rod is too long. It doesn't take much as the return port in the MC is small. The push rod can be adjusted without removing the MC from the car. Just unbolt it and push it to the side.

 

7. So remove the residual pressure valves, recheck the brake booster push rod length, and road test the car.

[NewZed]

Miles, how do these residual pressure valves work and what is their purpose?  Just trying to learn something.  Are they spring-loaded check valves, that release back to the reservoir at some set pressure?

 

Seems like an odd thing, maybe meant to work with certain parts.

[NewZed]

Found a few things. Interesting.  Seems likely for dreco.  Carry on.

 

http://www.mbmbrakeboosters.com/index.php?option=com_content&view=category&layout=blog&id=8&Itemid=16

 

http://www.championtrailers.com/content/remove-residual-valve-instructions.pdf

 

http://www.freeasestudyguides.com/residual-check-valve.html

[JMortensen]

Agree the rod is too long or pedal is misadjusted and the master can't fully retract. Had the same problem after misadjusting my clutch. More I drove, the more it slipped. Crack the bleeder and release the pressure and it was fine again.

[NewZed]

Typical MC at rest.  1980 280ZX Tokico brand.

Just noticed that the MC I showed appears to have residual pressure valves, aka "check valves".  I need to "upgrade" my brakes so I can learn more stuff.

[NewZed]

Now I really feel ignorant.  1976 shows a check valve also.  I have check valves.   I wonder if instead of removing these check valves, they should just be replaced with a lower force (pressure?) valve.  2 and 10 lbs seem to be the options.  Might be a fine detail, but if they've been using them in all models, they would seem to have some benefit.

 

Opinions seem to vary though - https://www.mico.com/sites/default/files/document-pdfs/81001017Residual%20Pressure%20Valve.pdf

 

I feel smarter now.  Good luck dreco.

Edited December 11, 2015 by NewZed

[Miles]

The residual check valves hold some pressure to keep the brake shoes close to the drum. Not needed in disk brakes.

 

However, every 280ZX and Wilwood MC I have bought had the residual check valves in the front and rear outlets of the MC.

[dreco]

I"m going to "check" on the check valve.  I did not remove them.

 

 

1. Tell us what year Z you have as there are differences in the hydraulic systems.

 

1972 240z

 

2. There are residual pressure valves in the outlets of the master cylinder. They look like a piece of rubber with a spring. Remove them.

 

Will Do

 

3. You should not have two proportioning valves (PV).  Having two PVs can cause problems other than what you have mentioned here. If you have for example,  a 72 240Z, and you have installed a PV in the rear then the stock PV should be removed. On later model Z cars you need to understand what the PV does before removing or modifying it.

 

My understanding it that the stock 'proportion valve' (engine compartment) in a '72'  really isnt one (only the restrictor valve in the rear)

 

4. The other question is why do you need a PV in the rear? 240SX rear calipers are  typically undersized relative to most so called "front brake upgrades". So unless you  are experimenting with high friction rear pads to fix front - rear balance issues a rear PV is not required with 240SX rear calipers.

 

Installed it prior to going with the big brake kit up front.  I only had the rear disc brakes at the time.

 

5. You can relieve pressure to the front or rear brake circuits by opening the respective bleeder on the MC if you need to limp home with dragging brakes.

 

its up on jack stands.

 

6. In my experience, the front brakes will lock up after a few stops if the booster push rod is too long. It doesn't take much as the return port in the MC is small. The push rod can be adjusted without removing the MC from the car. Just unbolt it and push it to the side.

 

I adjusted the rod all the way back just to make sure.  There is play in the rod.

 

7. So remove the residual pressure valves, recheck the brake booster push rod length, and road test the car.

 

I'll try the check Valves.   I wouldnt think it would be possible for the brake distribution block in the engine compartment could hold any residual pressure, but it's the only thing I havent replaced.

[dreco]

Thanks for all the info everyone!!  I'll takle this tonight.

[Miles]

The 72 240Z has the PV in the back (see picture). If you have installed a PV in the back then the stock PV should be eliminated or gutted out.

 

The device under the MC is a hydraulic differential pressure switch. It is not a PV. It also serves as a distribution block. If there is loss of pressure in the front or rear brake circuits a shuttle valve inside moves to the low pressure side blocking loss of fluid and at the same time grounding the wire that turns on the red brake light inside the car. The switch is NLA and there are no replacement parts for it. Every one of these I have opened up the shuttle was frozen in the neutral (middle) position. Don't mess with it. See pictures.

 

Push rod adjustment: The push rod should be almost touching the bottom of the cup at the end of the MC piston. For a starting point see attached picture. Too far out and your pedal will be long and grab when engaged. Too long your brakes will lock up. As a starting point, you have to measure  from the tip of the push rod to the face of the diamond shaped spacer 0.53" or 13.5mm. Mine usually ended up somewhere between 14mm to 15mm after several adjustments and road tests. When adjusting the push rod be careful not to drop the reaction disk inside the booster.

Edited December 11, 2015 by Miles

[Miles]

Miles, how do these residual pressure valves work and what is their purpose?  Just trying to learn something.  Are they spring-loaded check valves, that release back to the reservoir at some set pressure?

 

Seems like an odd thing, maybe meant to work with certain parts.

 

Check out Zcarnut's Wilwood thread  as it shows removal of the residual pressure valves. They block returning brake fluid through the action of a spring and rubber stopper when the returning fluid pressure drops to a preset level (e.g., 2lbs psi).

 

http://forums.hybridz.org/topic/102754-a-one-inch-diameter-brake-master-cylinder-for-the-s30-z-cars/

[dreco]

it was the little check valves, causing the issue.  Bled brakes and no more drag.  Now to put it all back together and test drive. 

 

FYI, the adjustable proportion vavle in the rear was put in to replace the stock pressure reducer valve.

 

Thanks for all the info everyone. 

[NewZed]

I wonder why Nissan puts check valves in the MC outlets of their four disc cars, if they're only needed for drum brakes.  Maybe not needed, but offer some small benefit?  A mystery.

[Miles]

I wonder why Nissan puts check valves in the MC outlets of their four disc cars, if they're only needed for drum brakes.  Maybe not needed, but offer some small benefit?  A mystery.

 

Perhaps the Nissan engineers wanted the pads to ride closer to the rotors for better brake feel???

 

Are the 240Z and 280ZX calipers of low drag design (piston retracts further into the cylinder)? That might explain the residual pressure valves in a master cylinder used for disk brakes. GM came out with low drag calipers in 1980 where the piston retracts further  into the cylinder.  This requires an extra squirt of fluid to get the piston moving towards the rotor. So they had to design a "fast fill" master cylinder to provide the extra fluid volume.  Google "fast fill master cylinder" and "low drag caliper."  Some of the hot rod guys using fast fill GM calipers resolve their "low brake pedal" problems by installing 10psi check valves in the brake system.