rear is locking up

[SHADY280]

so i finished my hb front disc swap, installed the 78/79 master cylinder. only problem is now, my rears are locking up in the wet and in gravel (hasnt been dry yet to test that) they are bled very well (pedal pump followed by gravity) and adjusted up nice and tight. do i need to install a brake bias valve??

[datzenmike]

Looks like it. They aren't legal for the street but touching the brakes on wet leaf covered corners and having it 'come around' is worse than worse. My 620 is about 16" above stock with 33s and would weight transfer forward on stops with resultant 2 foot skids at stop lights. Tired of getting stared at. When wet it would skate doing the same thing. I drove slower and slower and left 4-5 car lengths to try to avoid it. Eventually someone clued me in to a Willwood adjustable brake adjuster. I spliced it in line and bolted it to the fender and went some where safe and made some adjustments. I turned it almost the full 60% bias in addition to the Nissan NLSV and now the rears just begin to lock as the fronts do. With confidence I can jam them on without risk of loosing it on a corner. I can actually stop in a shorter distance now.

[banzai510(hainz)]

THIS MIGHT NOT BE YOUR CASE BUT WHEN MY DRUMS LOCK UP ITS USAULLY FROM A LEAK IN THE WHEEL CYLINDER.

 

mostly one than the other. maybe just let it where in when it gets dry.

 

or maybe your Returns springs are weak????? Im only guessing now.

[SHADY280]

rear brakes are new, i replaced one wheel cylinder due to a small leak behind the boot. i sprayed it all down and sanded the pads for a new surface. the tires being 3/32 tread prolly doesnt help, but it still shouldt lock like this. it was worse when the brakes werent adjusted up as nice.

[sssr20det510]

rear brakes out of adjustment will cause lock up, so will cheap shoes or out of round drums

[Cuts metal like mad]

adjust them a little looser maybe? 70-80% of your braking should be done with the front, so if loosening them down doesn't help a bias valve like Mike said is probably necessary

[banzai510(hainz)]

wet with bad tires lock up EZ

[SHADY280]

yeah looser they were worse, and the pedal was way low, and the e-brake didnt work. the drums are actually round, they dont catch and have the same amount of drag the whole way round. i know that it will lock up easier in the wet, but guess what, this is nw canada, its just like washington and rains 24/7/365. im gonna drive this in the snow, so i dont need it swapping ends on me. i know about the brakes, i am a mechanic by trade, i was just wondering if other ppl that completed this hb disc swap had simmilar experences.

[datzenmike]

Try going back to your '75? master.

[yello620]

Shady, i swapped to the HB v-6 front brakes, 79 rear axle housing(different bore wheel cylinders i think), and a 15/16" 280zx master cylinder. I have never experienced any lock up problems like this. I did NOT install an adjustable valve either.

 

I suspect a couple of things. First, if your front brakes have not had time to "seat in" The the rears are trying to do all the work still. If the "seating" does not help i would look at fluid pressures, i have seen the nissan NLSV stick before and severely limit the pressure to the front brakes.

[SHADY280]

the problem with the 280zx master is that it is meant for discs front and rear. that means the drums dont have the same drag as they should. the fronts arent seated yet, that is a good idea. i just dont have it insured and i did want to drive it with poor brakes. mike the 75 master was shot, so i chucked it, plus wouldnt it mess with the front discs???as far as the NLSV, it says its under the pass seat, but what controls it? angle of the truck? mines racked fwd a bit so could that make it apply more rearbias??? could i stick my adj. bias valve there instead? actaully it says there are 3. hmmmmm

Edited December 4, 2009 by SHADY280

[datzenmike]

The NLSV is tilted 10 degrees upward to the front when properly mounted. There is a steel ball inside that will roll 'up hill' under severe braking and close off or reduce the opening to the rear brake line to limit flow, and thus braking power, to prevent lock up.

 

When a heavy load is added, the frame lowers at the back increasing this 10 degree upward tilt at the front. This makes it harder for the steel ball to roll 'up hill' under severe braking and so allows more brake pressure to the back. The more weight in the back the more traction the rear wheels have so more braking can safely be applied to good advantage.

Edited December 4, 2009 by datzenmike

[72240z]

There are lots of people who upgrade just the fronts and go the the 15/16th ms and never have a prob with the rear drums or need a bias valve. I would look else where for the cause.

 

In a related case my d21 rears lock up after it rains. Taking a quick look it seems like the p/o installed the pads in reverse. Trucks got so many miles on it and everything is so damn seized and rusted I just deal with it.....

Edited December 5, 2009 by 72240z

[65dodge]

having a master that in disc font and back would not cause the the rear to lock up because disc break system uses less pressure than a drum break set up so there is somthing eles haping there

[datzenmike]

Yup, if it was a disc front drum rear on a vehicle with front and rear disc there would be a problem. Or more likely to be a problem. Maybe there's still air in the front.

 

Did you bleed the NLSV?

[SHADY280]

i see mike, so my truck tilted to the front actually helps the situation then. i have 280zx masters laying around everywhere, i just dont like to mix disc rear masters with drums, there is a valve that keeps fluid in the rear to help with reaction time of the drum brakes. dics masters dont have this. thats why im a little leary of doing that. i think id rather put a bias valve. soo many ways, so many answers

[ggzilla]

They aren't legal for the street

I wonder why Canada outlaws them. Certain Volvos in the 80s came factory equipped with an adjustable bias valve. It is needed when you change the Nissan-designed factory bias which is controlled by the brake design and the relative sizes of the pistons front/rear.

[yello620]

i see mike, so my truck tilted to the front actually helps the situation then. i have 280zx masters laying around everywhere, i just dont like to mix disc rear masters with drums, there is a valve that keeps fluid in the rear to help with reaction time of the drum brakes. dics masters dont have this. thats why im a little leary of doing that. i think id rather put a bias valve. soo many ways, so many answers

 

My 15/16" zx master has been on my 76 620 for about three years now, has yet to cause any problems.

 

The drum masters have a residual pressure valve installed in them, this valve is only there to keep a very small amount of pressure in the brake line. Usually less than 10psi. The idea of this line pressure is to keep air from entering past the the rear wheel cylinder seals when they retract. Thats it.

Disc calipers dont have that issue, they seal better/different.

 

If the zx master pressure valve idea is a bother, you could always install an aftermarket residual pressure valve. They are available in 2psi for discs and 10 psi for drums. They are generally only needed when the master cylinder is lower than the calipers/wheel cylinders, like on old hot rods with the master cylinder under the floor on the frame rail.

[hawaii_620]

I have a similar problem with my rear drums after doing the D21 disc conversion to the front. I keep reading about the NLSV and about bleeding it, but I don't have one on my truck since my truck is a 75. But my rears lock occasionally, I can actually hear a "clunk" and then feel the drag immediately after. Going in reverse unlocks it.

 

I must still have air trapped in the system or my brakes are "broken in" yet, because it still takes a good distance to come to a stop. I definitely can't make it skid.

 

Still reading all these past threads and trying to figure out what needs to be done to make my brakes work... like a brake system should work.

[datzenmike]

Take the drums off the back and look for something loose, they shouldn't do that. Also adjust them. Drum brakes require lots of wasted peddle travel if the shoes have a long distance to travel to reach the drum. Set and adjust the E brake after this.

 

Most of the stopping is done by the fronts. (drum or disk) The engine and cab weight and the shift of weight forward during braking, increases the traction on the front tires so more brake can be applied to them.

 

Did you replace the master cylinder? Your truck has disc brake on the front now and needs a larger MC for this. If you still have the old one it will have the wrong residual pressure valve in it as well.

[Hoosier77]

The NLSV is tilted 10 degrees upward to the front when properly mounted. There is a steel ball inside that will roll 'up hill' under severe braking and close off or reduce the opening to the rear brake line to limit flow, and thus braking power, to prevent lock up.

 

When a heavy load is added, the frame lowers at the back increasing this 10 degree upward tilt at the front. This makes it harder for the steel ball to roll 'up hill' under severe braking and so allows more brake pressure to the back. The more weight in the back the more traction the rear wheels have so more braking can safely be applied to good advantage.

 

shit so when i loered my truck with a bit of front rake i lost some braking in the rear? damn, how would you correct for this with the truck still being lowered? can you remove the valve all together?

[datzenmike]

If the brakes are set properly, they should all lock at about the same time under hard braking. When weight is added to the back more braking effort can be applied to the rears because of the increased traction on the rear tires. The extra weight will lower the frame at the back which will increase the 'tilt' of the NLSV making it harder for the ball to roll up hill and reduce the brake pressure. The reverse is true... less weight equals less braking to prevent rear lock up.

 

I would look at tilting the NLSV up slightly and bolting it to the frame. Beware, you don't want too much brake pressure getting to the rears or they will lock.

 

Set and properly adjust the rear brakes and take the truck out on flat bare pavement and safely apply more and more brake until it locks. Get out and inspect the tire marks. The rears should lock about the same spot as the fronts. Rear lock before the fronts must be avoided! When the rears lock first the back end will want to 'come around' on you like massive over steer. If there is no rear tire marks maybe the NLSV could be tilted up. If there is equal but faint marks from the back I would leave it.