RE: Locking Brakes

[PATRICK_KELLY@HP-Andover-om3.om.hp.com]

Item Subject: Text_1
     I have been chasing a similar mysterious brake problem on my '875kSQ 
     for a few months.  Typically the self-locking when hot brake problem 
     has been cured by replacing the master cylinder.  Rumor has it that 
     when replacing brake pads, if the caliper pistions are pushed in with 
     out opening the bleed valve, all the gunk settled in the calipers gets 
     pushed back to the MC and can block off certain orifaces (sp? orifi?). 
     When the fluid heats up and expands, it can't expand through the 
     blocked MC so it clamps the caliper.  The symptoms on my car were 
     moslty just a hard peddle which I at first thought was a hydraulic 
     system problem so I drove around for a week with a guage plumbed in 
     the my booster but when the car got hot and the pedal went hard, 
     hydraulic pressure stayed steady at 2200psi.  After hearing about 
     replacing the MC on the list, I decided to try a thourogh brake fluid 
     flush first.  The problem almost went away and I didn't really notice 
     any problems for the amount I drove everyday.  But on longer weekend 
     trips the hard pedal would still come back.  It always happened after 
     the radiator fan came on and heated up the MC and resevoir.  So while 
     staring at the works one 90+ degree August day, I wondered what 
     happens to the air in the top of the resevoir when it gets hot and 
     expands, it should vent out through the cap but what if it didn't.  
     The next day I was taking a 5 hour drive to northern Vermont.  An hour 
     into the drive the the pedal went hard.  I stopped and took the cap 
     off the very hot brake resevoir and immediatly replaced it and drove 
     off. I drove the next 4 hours with no problem.  This has worked 
     several times since then.  So my theory is this: The resevoir cap did 
     not vent pressure built up in the resevoir which then put a small 
     amount of pressure into the brake system causing the pedal to become 
     hard (no "slack" to take up in the system).  This could, in theory, 
     become extreme enough to cause significant brake drag.  It took a 
     while for me to believe this is whats happening and the weather here 
     in the northeast is cold enough so I haven't done any fixes or tests.  
     Any thoughts on this half baked resevoir idea (pun intended), I'd like 
     to hear 'em.  I also noticed on the Alldata website the other day that 
     there is a TSB from August of '88 #4688T03 - Brake-Insufficient 
     Ventilation.  I don't have the TSB's so I would appreciate any lister 
     with the TSB's replying if this is relevant.  
     
     Patrick Kelly
     Dracut MA
     '87 5kSQ