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Brakes get Hot,Drag,then Lock Up?

9K views 53 replies 14 participants last post by  BRN2RUN 
#1 ·
My 69 Chevelle has a Right Stuff Disc Brake Conversion kit . After driving for few minutes the brakes start to drag, then you can smell them and if you stop they will lock up . After about ten minutes they cool down and you can drive again . Any ideas ? This is pretty frustrating . Thanks!
 
#7 ·
What they said about the pushrod. The plunger in the M/C needs to be able to get ALL the back to rest position. If it doesn't a tiny little relief port is covered up and pressure builds. This is a very common issue these days with all the aftermarket being used.
 
#9 · (Edited)
Well I adjusted the push rod and it didn't make a difference . Even after separating the the master cylinder and booster TWO front brakes remain locked . I even tried putting two small washers between the Booster and MC, no difference. Any ideas? Remember it's both front brakes that lock! After a cool down they release.
 
#10 ·
Where is your proportioning valve located? On the MC or on the frame in the original location?
Do you have headers?
If the proportioning valve is on the frame and you have headers, the heat from the headers may be heating up the P-valve, which means the fluid is getting hot, which means the brakes will lock up. Then release when the fluid cools down.
It happened to me after adding headers to my Elky. I had to relocate the P-valve to the MC.
 
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#11 ·
Is the piston in the master fully retracting? Also if you bench bled the master and over stroked it, the seals can invert and prevent the piston from actuating properly.
 
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#13 ·
How can I tell if the piston in the MC is fully retracting? When I got home and jacked the front of the car up, you couldn't turn the wheels by hand. I separated the MC from the booster and still the wheels where locked . It wasn't till they cooled down that I could turn the wheels!
 
#14 ·
2 ways.
1) with the master cylinder pulled away from the booster, is the piston fully moved to the rear of the master closest to the booster?
2) with someone watching the master with its lid off and someone gently operating the brake pedal you should be able to see a small "geyser" erupt from the bottom of the master each time the brake is pushed and released. That is fluid entering and leaving the piston port of the master. It's a small hole at the bottom of each reservoir.
 
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#16 ·
One other thing to check. Make sure that your caliper slide pins are clean and LIGHTLY luricated with brake system grease. I once saw a set that were painted and when the paint got hot the pads would hang up on the paint.
 
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#23 ·
I've never heard of this causing this problem. The piston in the bore is immersed in the bottom of the fluid reservoir at all times, it doesn't care how much fluid is sitting above it.


================================

BRN2RUN, tell us more about this "kit". Did it include a new master cylinder? If it did, consult the maker.

Drum brake setups have a residual pressure valve in the system to keep about 5 PSI maintained to the drum cylinder to help overcome the springs that pull back the shoes when you let go of the brakes. This cannot be used with disc brake calipers.
 
#18 ·
Still chasing it!
Didn't see the small geyser when pushing the pedal down ! = bad MC ?

Didn't even have to drive car for front brakes to lock . I just went out and raised the front of the car , both wheels turn freely. After a few minutes wheels get hard to spin . Insulated the PV and tried again, same thing, wheels get hard to spin and pedal is real hard and car was never moved! :frown2:
 
#19 ·
If it were me I would be pulling the master and tearing it down. Piston to pin clearance or piston not fully retracting is my suspicion.
 
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#21 ·
X2 that’s what I would double and triple check you need a small gap in there.
There is a neat tool that I used to set the pushrod length. $25 on amazon
What MC are you using with the right stuff brakes? The AC delco Vette style can be bought cheap.
Never had a issue with RSB myself but I did have to center the calipers over the rotors properly on the rears. The provided spacers wouldn’t work.
 
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#20 ·
Just one more item. Check the delay valve below the master and make sure it's little valve moves freely. It should have a black button on its firewall side that you can depress and it should move freely.
 
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#22 · (Edited by Moderator)

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#25 ·
Last year I was having trouble getting my rear brakes to bleed after a total brake system rebuild. I replaced or restored every component in the brake system except the proportioning valve and the brake pedal. I was not getting the geysers either.
Anyway I chased the problem down to the push rod being too long. I did not use the special tool but did use a straight edge and a dial caliper to determine the gap. IIRC had to remove .060 from the push rod to get the proper gap. After doing so I was able to get the geysers in the master reservoirs.
Here is a thread if you care to read it. My gap problem is discovered around post #55 or so

https://www.chevelles.com/forums/16-brakes-suspension-steering/1063226-rear-brakes-wont-bleed-3.html
 
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#26 ·
OK Guys after rereading all the helpful hints I wen't back to square one ! This time "before " starting the car I pulled the MC all the way back to the last thread and then started the car and l et it get hot . Wheels no longer are locked ! I had used the washer test earlier but evidently that didn't create enough space . So at least some progress !!!
So what might seem like a silly question....when adjusting the pushrod .
Do I turn the pin head on the end of the rod OR the nut behind it ? The rod is recessed pretty well back in the MC .
Thanks for all the help !
 
#27 ·
Post a picture. Sounds like you have a Bendix style booster. Want to be sure. Did you watch the video?
 
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#28 ·
If you are talking about the push rod from the brake booster to the master cylinder that is not adjustable. At least on a factory brake booster. I had to remove mine from the booster and carefully grind off .060 on my bench grinder.
 
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#29 ·
He has parts from Right Stuff. May be a Bendix style booster.
 
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#31 ·
Gently see if the pushrod can be removed. SOME WILL SOME WON'T. You can then see how it shortens. The lock nut needs to be loosened so you can turn the pin in. Then retighten. Post a clear pic of the inside of the back of the master. Want to make sure your piston is retracting. Something looks off but it may just be the pictures?????
 
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#39 ·
Gently see if the pushrod can be removed.

DON'T! Pulling the push rod of BENDIX booster (adjustable pin) may cause the reaction disc to fell at the bottom of the booster (the reaction disc can stick to the push rod and fell off the power piston). If that happens, you need to disassemble the booster to fetch it and reinstall it. Not a problem with DELCO boosters.




The BENDIX booster has always the "long" push rod by design. The fact that the "washer" trick didn't work tells me that you must have a m/c with a shallow recess in its piston, which is made for a short push rod. No adjustment can compensate for that, there is over 1" of difference between the two.


You need to replace the m/c (or at least its piston) with one with a deep recess, or replace your booster with a DELCO model, with a short push rod of course (long & short push rod are interchangeable on any DELCO booster). There are no "short" push rods for BENDIX boosters.





Left: Deep recess. (About 1" deep) Right: Shallow recess. (Almost flush)
 
#32 ·
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#33 ·
Thats a great Link, I'll continue to read through it!
The push rod doesn't seem to be removable .
Couldn't really get a "clear" picture of the back end of the MC while it's still attached , but just by feeling it seems to be all the way back. Remember brakes didn't lock after pulling back the MC.
Thank You!
 
#34 ·
Just for clarity, did the brakes work correctly at one time prior to now?
 
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