720 Brake Problems

[mcmmech]

I have a '84 Nissan 720 311K miles and a brake problem. At low speeds, steep down hills and the rain the rear brakes lock up, even with extremely steady pressure on the pedal. I have replaced all components (master cylinder, front calipers, rear wheel cylinders, and I even swapped the NLSV valve with one from the junk yard) still no change.

UPDATE: I have disks on the front, drums on the rear. I looked at the system and the front operates SEPARATELY from the rear. (one line comes out of the master cylinder, splits and goes to each of the front wheels: another comes off, goes towards the rear, through the NLSV valve then splits and goes to each of the rear wheels)....one thing i haven't tried is new drums.......i'll put a dial indicator on 'em...see what the deal is...

Thanks for all the suggestions.....keep em comin'!
Modified by mcmmech at 5:09 AM 11/18/2009

[wesradford]

I had a 1975 620. Had four wheel drum brakes. Did a similar thing...locked up with steady soft pressure...sometimes... other times it worked fine. After many, many inspections I noticed an ever so slight leak on one of the wheel cylinders. After rebuilding the cylinder and replacing the pads and cleaning the drum with some sort of solvent (can't remember what kind), the problem was solved.

This was a TINY leak....

[flinterman2000]

When I got my 85, it had drums all around, a little play on the brake pedal. The left front and rear would lock up once past the play at low speeds. I changed the booster using one from a B14 Sentra and got rid of the play but it still locked up as you touched the pedal. I changed the front drums and installed discs from a nissan caravan. The front stopped locking but the rear continued. I changed the brake fluid and adjusted the E brake and now tapping hard on the pedal locks it up. I also noticed the drum needs cutting as its not a perfect circle.

[PEZi]

I changed the front drums and installed discs from a nissan caravan.

coulda just got 'em off another 720 lol

[flinterman2000]

Its an upgrade. the disc is .25 bigger, the calipers carry two pistons, they are ventilated and use larger disc pads but use the same wheel bearings but a larger steering end. All I did was remove the turn radius stops and shim the hole for the end.

[dfunk]

Because this is one of the first things that comes up on google, I wanted to solve this because I ran into this problem with my 720 and have been dealing with this MOFO problem for the past two weeks (while driving it). My truck was doing fine once I adjusted the star adjuster for about 50 miles and then it would revert back to locking up under light pressure. I replaced every soring and retainer + wheel cylinders and no help. I bought drum shoes 3 times in 2 weeks. The solution was my parking brake. One of them was frozen and the method used to auto-adjust in these trucks is to pull the parking brake not driving backwards and stopping. DRIVING BACKWARDS AND STOPPING DOES NOT WORK. I have verified this by going to a brake and clutch shop and inquiring about this. He pulled out this crazy Nissan book that had a picture of my exact brakes on it and said that you pull the parking brake to engage the star wheel (self adjuster). Remember, make sure your parking brake is functioning and both sides are working correctly and adjusted correct.

[torranosaurus]

Hello.
I just bought an 86 2wd std. plain jane truck in excellent 100% original condition. It is also doing the same thing, really badly too.
Are you saying to adjust we drive it, I assume at low speed and pull the parking brake slightly to adjust, or would that be done to unstick the brake? The truck has 89000 miles and sat in storage for years and hasn't been driven much. I just bought it.

[torranosaurus]

Anyone?
Still having the problem.