crazyoctopus Posted April 27, 2010 Share Posted April 27, 2010 (edited) to quote JMotensen from 2006 on this thread http://forums.hybrid...%20locking%20up drive the brakes progressively start locking up worse and worse until the car just won't drive anymore. If I let it sit for an hour then I can drive again, but then the same thing happens!" It's come up here a couple times, and happened to me on my clutch. Went to a track day and the more I drove the more the clutch slipped, same deal.The fluid in the master expands when it gets hot and this trys to force the master cylinder cups past the fluid return hole. Once it expands to the point where the return port is blocked off, the pressure in the master can't be released back into the reservoir. You need slack in there so that the master can fully release even when the fluid gets hot. my question is what should i be adjusting? the push rod in the vacuum booster, or the rod attached to the brake pedal itself? To me it seems that when i adjust the rod attached to the brake pedal all i am doing is just changing the location of the brake pedal, not changing the engagement point on the master. After I installed the 15/16th brake master I have been messing with the vacuum booster's push rod length so that I can get a good amount of pedal feel, and right when I had it perfect this started happening. So I backed out the rod a 1/2 turn, and while the brake pedal had worse feel it seemed like the problem had gone away, but it didn't. my brake set-up if needed. 240sx rear disc conversion stock front calipers SS lines all around 15/16th brake master cylinder Edited April 27, 2010 by crazyoctopus Quote Link to comment Share on other sites More sharing options...
stravi757 Posted April 27, 2010 Share Posted April 27, 2010 Had the same problem, I tightened the push rod a few turns, never had the problem again. Im running 240sx and toy vented. Quote Link to comment Share on other sites More sharing options...
JMortensen Posted April 27, 2010 Share Posted April 27, 2010 (edited) Oops. Didn't see myself quoted there... I would adjust the push rod on the pedal and see if that helps, if not then pull the master off and adjust the other one between the booster and the master. Edited April 27, 2010 by JMortensen Quote Link to comment Share on other sites More sharing options...
crazyoctopus Posted April 27, 2010 Author Share Posted April 27, 2010 stravi is your pedal weak? because that was what i was feeling with the vacuum booster push rod turned back, even at the current position the pedal pressure is just... lacking. and only really kicks in at the bottom; i can still lock up the tyres under an emergency stop, but only just. tomorrow i'll max and min the pedal to see if that helps, if not i guess i'll keep trying to tighten down the vacuum booster pushrod. 1 Quote Link to comment Share on other sites More sharing options...
stravi757 Posted April 27, 2010 Share Posted April 27, 2010 stravi is your pedal weak? because that was what i was feeling with the vacuum booster push rod turned back, even at the current position the pedal pressure is just... lacking. and only really kicks in at the bottom; i can still lock up the tyres under an emergency stop, but only just. tomorrow i'll max and min the pedal to see if that helps, if not i guess i'll keep trying to tighten down the vacuum booster pushrod. Are you sure your rear calipers are bled all the way? I remember my rears wouldn't bleed properly until I actually unbolted the calipers to turn them at a different angle while bleeding them to get all of the air out. Quote Link to comment Share on other sites More sharing options...
JMortensen Posted April 27, 2010 Share Posted April 27, 2010 stravi is your pedal weak? because that was what i was feeling with the vacuum booster push rod turned back, even at the current position the pedal pressure is just... lacking. and only really kicks in at the bottom; i can still lock up the tyres under an emergency stop, but only just. Weak pedal meaning soft? If so, that sounds like reaction disk. You might have a couple things going on here... Quote Link to comment Share on other sites More sharing options...
stravi757 Posted April 27, 2010 Share Posted April 27, 2010 Weak pedal meaning soft? If so, that sounds like reaction disk. You might have a couple things going on here... +1 also on the disk. Quote Link to comment Share on other sites More sharing options...
crazyoctopus Posted April 27, 2010 Author Share Posted April 27, 2010 weak in a sense that it is as stiff as my fit's pedal but doesnt have the stopping force i'd thought until I get near the end of the travel; I cant remember what the car felt like when it had the 7/8th. The car is going up on jack stands tomorrow for a engine and coolant flush, as well as some other things, so i'll disconnect the brakes and vacuum booster while it is up there. the rear brakes were jetting out solid fluid last time i bled the system (last weekish) but i'll put a bottle on it and see if that make a difference, as well as check if the reaction disc is in proper order. Quote Link to comment Share on other sites More sharing options...
crazyoctopus Posted April 27, 2010 Author Share Posted April 27, 2010 alright so here is the process to make sure all the tests were equal I made sure that the brakes were squealing (carb metalic brakes) when I pulled into the driveway. if the MC was detached from the car i can push the car back and forth with little effort, signifying that the brakes are not grabbing onto the rotor. 1. adjusted the pedal to the maximum; with the rubber pad off it went from 7.75" to 8.5" to the firewall. -This produced no change what-so-ever, so that pretty much means that the brake adjustment behind the firewall only adjusts height of the pedal, not the actuation point like with the clutch pedal. push test resulted in nothing, car was stuck. 2. adjusted the vacuum booster (VB) push rod from 21.97mm (from tip to front of vacuum booster, not the aluminum bracket) to 20.88mm. -This produced a slight change in so that it took longer for the brakes to seize to the rotor, and when I push tested the car it took some effort, but still a vast improvement. 3. adjusted the VB push rod to from 20.88mm to 19.98mm (the distance from the tip's edge to the VB was aprox. 19mm, measured with a 19mm headed bolt). -massive change and the issue is solved. the push test resulted in the same effort needed to push the car as it did when the MC was out. I measured to the vacuum booster's wall because that aluminium plate god in the way of getting the vice grips in there to hold the push rod in place; that plate measures out to 10mm. I remember reading somewhere that the VB push rod had to be something like 8-9mm to the Al. plate. So my measurement fits in that limit. I am going to pull teh MC one last time and see if i have a deep socket that measures out to either 9mm or 19mm to make sure that I am spot on, and not just barely there. Thanks for the advice, it helped a ton! Quote Link to comment Share on other sites More sharing options...
crazyoctopus Posted April 27, 2010 Author Share Posted April 27, 2010 here are a couple of pictures for reference. Quote Link to comment Share on other sites More sharing options...
crazyoctopus Posted April 28, 2010 Author Share Posted April 28, 2010 pedal isnt as stiff as i'd like it to be, but at least the brakes dont lock up when i dont want them to; i'll be doing some tests to see if i can still do emergency stops tomorrow. Quote Link to comment Share on other sites More sharing options...
Recommended Posts
Join the conversation
You can post now and register later. If you have an account, sign in now to post with your account.