ditto64 Posted October 24, 2016 Share Posted October 24, 2016 So I've done the 4-pot Toyota caliper upgrade to my '78 280z, and it worked fine for a couple years -- no issues. Fresh pads. All other brake hardware is stock, except for a new stock master. Recently, after 30 or more minutes of driving, the brakes will start grabbing while I'm on the road. The pedal gets considerably harder to depress. So far I've checked for kinked hardlines, confirmed there are no leaks, and taken the calipers off and examined with no success. I know both the front and rear driver's side brakes are grabbing -- I'm not 100% sure it is all 4, but I think so. The brakes heat up to the point of smoking, and I feel strong resistance when driving and the brakes engage. My only thoughts are that I've got a bad brake booster... anyone else experience this problem or have any insights? Thanks, Dave Quote Link to comment Share on other sites More sharing options...
NewZed Posted October 24, 2016 Share Posted October 24, 2016 except for a new stock master. Recently, after 30 or more minutes of driving, the brakes will start grabbing while I'm on the road. .. anyone else experience this problem or have any insights? Hard to tell what the timing is on your description but if "recently" means essentially right after you installed the new MC, then you probably have the rod from the booster to the MC adjusted too long. It closes the return hole for the fluid, and the fluid heats up and expands in the lines, actuating the brakes. It's actually a fairly common problem. You may not have adjusted the rod when youdid the swap but the new MC might have its seat for the rod closer than the old one. If "recently" meant months after the new MC was installed and it's been fine for many miles, then never mind the above. Quote Link to comment Share on other sites More sharing options...
grillhands Posted October 24, 2016 Share Posted October 24, 2016 Not sure if this helps but check the check valve on the vac line going to the master cylinder. It could be bad and allowing positive pressure in the system. That would make your pedal feel like a brick Quote Link to comment Share on other sites More sharing options...
Miles Posted October 25, 2016 Share Posted October 25, 2016 Most likely the MC push rod is adjusted too far out which will block the return port in the MC. With the return port blocked pressure will build up with each application of the brakes until they drag/lockup. Test: Open the bleeder ports on the MC when the brakes drag. This will release the pressure and unlock the brakes demonstrating that the push rod is adjusted too long. You can adjust the push rod without removing the MC. Remove the retaining nuts on the MC and push it aside to access the push rod. Make small incremental adjustments followed by road test each time. Carry a wrench that fits the bleeders on each road test in case you lockup the brakes. If you back off the push rod too far your brake pedal travel will be long and grab. All you need is about 1mm clearance between the push rod and the MC piston. Research the brake section as this problem has been discussed many many times. Quote Link to comment Share on other sites More sharing options...
74_5.0L_Z Posted October 25, 2016 Share Posted October 25, 2016 Did you overfill the master cylinder fluid reservoir? In preparing my car to run at Sebring the first time, I replaced the brake fluid with Motul 600 and added cooling ducts to the front brakes. Unfortunately, during the brake bleeding I overfilled the rear reservoir. About 15 minutes into the first session, the rear brakes overheated and I was not even driving the car all that hard. Inspecting the car in the paddock after the session, I discovered the overfilled rear reservoir was pressurized. As the brake fluid heated, it tried to expand but because the reservoir was full it had no room to grow. The expansion caused the rear brakes to drag slightly and continuously. Eventually, the rear brakes overheated. Quote Link to comment Share on other sites More sharing options...
ditto64 Posted October 25, 2016 Author Share Posted October 25, 2016 I will try all of these solutions once I get a chance and report back. The new MC was installed a few years ago, and this is a recent problem. I haven't had this issue in the past, so I'm skeptical that the pushrod is adjusted incorrectly. Quote Link to comment Share on other sites More sharing options...
JMortensen Posted October 25, 2016 Share Posted October 25, 2016 Drive it until it happens, then crack the brake bleeders. If that solves the brake lockup problem, then it has to have something to do with residual pressure in the system. I hadn't heard of Dan's issue with overfilling, but I've experienced the other one firsthand with a clutch system, but same problem. Thought it was a good idea to take the slack out of the pedal to master adjustment. The more I drove, the more the clutch fluid heated up, the more it slipped. Quote Link to comment Share on other sites More sharing options...
bfr812 Posted October 30, 2016 Share Posted October 30, 2016 I had the same problem on my 73 240z. My brake fluid had somehow been contaminated while in storage for 20years. I could bleed the brakes just fine but as soon as they heated up they would drag to the point of lock up. I finally pulled the master there was gel in it as well as in the booster. i replaced the master and cleaned the booster and then flushed the system. Good as new!! clean the mixing valve while you're at it. (NOTE: DON'T PULL THE ROD OUT OF THE BOOSTER. once it's back together adjust the rod from inside. look for posts on this. Good luck! B Quote Link to comment Share on other sites More sharing options...
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