Brakes locking up. Need Help!!

[murdrdatsoon72]

I have the usual 280zx front strut and disc brake setup on my 72 wagon. My issue is when I first drive off the brake pedal operation is soft. After about 20 minutes of normal driving the brakes start locking up and the brake pedal gets really hard. I rebuilt the front calipers  (sand/polished piston and bore) new seals and bled Master. I thought at first it was just the front brakes but when I checked the rears (drums) they too were locking up. If I let the car cool down, the brake pedal is soft again until I start driving it for awhile then they lock up. I am running the 280zx dual Master and just picked up a porportioning valve from a 81 280zx. The problem with this setup is my original setup has 5 lines going into it plus brake switch and the 81 280zx only has 4 and no brake light switch.

I was told that the wagon does not have a porportioning valve rather a hydraulic pressure switch in the brake system.

Can anybody give me some suggestions on this problem?

[Crashtd420]

Only thing that pops into my head is are you over full in the reservoir?

 

I had an atv do it to me, because it was to full it heated up and it locked the rear tire....

 

In my case I cracked the bleeder to relieve the extra pressure and everything worked good, later drained some fluid from the reservoir...

And when I did crack the bleeder fluid shot under some force so be careful...

[mklotz70]

Did you mess with the push rod at all?  It sounds like your m/c's piston is not fully returning to where it should.  It can either be the push rod is adjusted too tight or you have a bad m/c.  When it sits, the fluid probably seeps back into the reservoir.

 

Do your self a favor and get a switch that works on the brake pedal.  That way you get brake lights as soon as you touch the pedal instead of after the pressure gets high enough.  You have to extend the wires into the cabin, but it's definitely worth it.  Hmmm.....I see it's a 510.....I thought they already had the pedal switch.  If the switch in the engine compartment has only one wire, it's a pressure differential switch that turns on the ebrake light on the dash when one of the circuits loses fluid/leaks.

[banzai510(hainz)]

I had a bad Master that would pump up everytime I hit the brake and not release the pressure unless I hit the blled screw to release it. It was all 4 wheels.

 

It might be your proplem or not.

[Dolomite]

I’m with mklotz on this. You need about 1/8” of play between the rod end and the piston in the master. I just went through this after getting my new 510. I put in a bit of play and now everything is kosher.

[murdrdatsoon72]

Thank you all for your advise. I will check these suggestions. I did not adjust the brake play, thought about it. Has anyone installed a 280zx proportioning valve on their wagons or should I just keep the original pressure switch? The 280zx MC was bought new about a year ago. Car normally sits in garage (3 years now) because its not street legal yet. Also this braking problem needs to be resolved.

[datzenmike]

I'm not convinced there is a proportioning valve. Just the brake warning light. The proportioning or 'bias' is easily built into the brake system by engineering the sizes of the calipers on the back to complement the fronts. Simply put.... why have rear calipers that are too large to begin with that need a proportioning valve????

 

 

Also the rear system should have a drum brake residual valve in the MC. If a 280zx, then you will have a disc brake residual valve in there..

[murdrdatsoon72]

Problem solved!! Mklotz and Dolomite was right. Thanks Bros!! The pushrod was too far extended, I backed down the rod to 1/8" and the brakes work perfect without locking up!! 

 

Anybody interested in an 81 280zx proportioning valve (used) with all connectors and clamps? :thumbup: Just bought it off Ebay!

[mklotz70]

Glad to hear you got it figured out. :)

[EWD]

On 6/12/2018 at 4:56 AM, mklotz70 said:

Did you mess with the push rod at all?  It sounds like your m/c's piston is not fully returning to where it should.  It can either be the push rod is adjusted too tight or you have a bad m/c.  When it sits, the fluid probably seeps back into the reservoir.

 

Do your self a favor and get a switch that works on the brake pedal.  That way you get brake lights as soon as you touch the pedal instead of after the pressure gets high enough.  You have to extend the wires into the cabin, but it's definitely worth it.  Hmmm.....I see it's a 510.....I thought they already had the pedal switch.  If the switch in the engine compartment has only one wire, it's a pressure differential switch that turns on the ebrake light on the dash when one of the circuits loses fluid/leaks.

Thank you for this! I read this last night and couldn’t wait to make some adjustments. I had the same issue and thought it had to do with my hydraulic hand brake or the 1” M/C I installed. After adjusting the rod one direction the brakes started seizing after 2 minutes of driving. Went the other way with it (as far as it would go toward the driver seat) and have had no more issues. This is a HUGE win for me and my 4 door.

[mklotz70]

Glad to hear that it helped. :)

 

[dadymurano]

I put a booster and master cylinder from a 710 on my 620 out of frustration returning  620 master cylinders that were bad out of the box. (rebuilt)

 I had to modify the plunger linkage for it to work. Ran into this problem also.

 

 Still got drum brakes on all four wheels and they work really well. Maybe one day I'll go with disks but so far I'm OK.

[wayno]

Your rod connected to the brake pedal going into the booster/brake master might be adjusted wrong, try loosening it a little, it needs a little bit of play/slack.

[Gery]

My dash brake light stopped going on at start-up and when the hand brake is engaged.  I used a multimeter on the line to the handbrake switch and got no voltage.  Possibly a burnt bulb  on the gauge?  Before having to remove the dash instrument panel, is there anything else I can try?  If this was only a handbrake issue I would be less concerned....but I don’t want to drive around without a warning light for loss of brake fluid. All six fuses in the main fuse box test as good.  I checked the reservoirs...there are no float switches on the lids...so I couldn’t test the light that way.

 

Oh yeah, and on the subject of brakes.... how much difference do performance disc systems make over stock disc systems.  I’m talking front only....rears are drums.

Edited November 13, 2019 by Gery
Additional information

[datzenmike]

Power to the brake, charge and oil pressure light comes from the same fuse so if the other two are working then you brake bulb is burned out or not making contact. You should be able to read some small voltage at the hand brake switch. This is the '79 620?? It seems that it doesn't have a warning for brake fluid loss, only to warn if the hand brake is on.

[Gery]

Thanks!    (‘78 kc 620).  That helps a lot.  I always appreciate your responses. 🙂

[datzenmike]

I thought all dual master cylinders had the pressure differential warning light. When you step on the brake there will be, more or less, equal pressure in both front and back brake circuits, so the switch senses nothing. If there was a leak in either the front or the back, pressure on the one side would activate the switch turning on the warning light.