Brakes locking up.

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Jacmifish

Jacmifish

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A few years ago I bought a front disc conversion kit from Mancini Racing and installed it prior to running with my rebuilt /6. At some point when initially running with the new motor the brakes seem to heat and stick while driving. The only way to relieve pressure was by cracking the lines open at the master cylinder. I have been doing a lot of other things lately to the car and am now trying to tune it again. The car seem to run okay prior to bleeding the brakes and after bleeding them they seem to heat up and lock up. So bad that it will sit in drive and not move. I considered making a heat shield for the master cylinder because I have wrapped headers. Does anyone have any ideas as to why this might be happening?

This problem needless to say makes it much more difficult to tune the car haha.
 
Make sure the brake push-rod is returning enuff to let the Master cylinder return to it's stop..
If it appears fine,, replace the M/C..

hope it helps
 
Jacmifish said:
The car seem to run okay prior to bleeding the brakes and after bleeding them they seem to heat up and lock up.
Click to expand...

Well, I'm not 100% clear on what you are saying here.

Is this just the front disks locking up?

Do they lock up without pushing on the pedal? (which is kinda what I got from the way you worded it)
 
Bad Sport said:
Well, I'm not 100% clear on what you are saying here.

Is this just the front disks locking up?

Do they lock up without pushing on the pedal? (which is kinda what I got from the way you worded it)
Click to expand...

They are the front discs locking. A few days ago I was messing around driving the car and I knew the brakes needed to be bled; they felt a little spongy but worked fine and didn't lock up. After bleeding the brakes today the brakes felt stiffer and after I tried driving the car the would lock up without me pushing on the pedal.


inertia said:
Make sure the brake push-rod is returning enuff to let the Master cylinder return to it's stop..
If it appears fine,, replace the M/C..

hope it helps
Click to expand...


There seems to be a little travel the pedal could come up further, however while in drive and I push the brake pedal up as far as it will go and it will not release the brakes.

I plan to check and see after the motor cools down later if the pressure has released; if the pressure hasn't released then I will crack open the lines on the master cylinder to relieve the pressure.
 
When you did the front brakes did you replace the rubber lines?

Sometimes those lines can collapse inside and you can get fluid through them one way because of the pressure applied when using the pedal, but it doesn't have enough pressure to relax and let the fluid back off.
 
Bad Sport said:
When you did the front brakes did you replace the rubber lines?

Sometimes those lines can collapse inside and you can get fluid through them one way because of the pressure applied when using the pedal, but it doesn't have enough pressure to relax and let the fluid back off.
Click to expand...

also this..... ^^^^^
 
By what I read, the issue started immediately after the swap. Is that correct? Did you change the master cylinder also or just convert to disc?
 
I'm thinking if the brakes release by cracking the line at the M/C,, that a rubber hose will only hold pressure in an localized area,,

I think the M/C rods outta adjustment, if you changed M/C.. or if it's the original Master cyl, it still has a residual pressure valve in the front system (re Dubob).., or the M/c cups have swollen..

hope it helps
 
inertia said:
I'm thinking if the brakes release by cracking the line at the M/C,, that a rubber hose will only hold pressure in an localized area,,

I think the M/C cups are swollen,, or the Master cyl still has a residual pressure valve in the front system..

hope it helps
Click to expand...

I bet you're right, I forgot he said he was cracking the line at the MC.
 
inertia said:
I'm thinking if the brakes release by cracking the line at the M/C,, that a rubber hose will only hold pressure in an localized area,,

I think the M/C rods outta adjustment, if you changed M/C.. or if it's the original Master cyl, it still has a residual pressure valve in the front system (re Dubob).., or the M/c cups have swollen..

hope it helps
Click to expand...

You hit dead on my thoughts about the residual valve still being installed.
 
The residual valve would never be able to lock up the brakes. You should remove or puncture it if present, however.

What is PROBABLY wrong is that either there is a wrong piston / other parts in the master, or else the pushrod is TOO LONG and not allowing the piston to return BACK past the port in the floor of the cylinder reservoir
 
I would jack up the front right after you brought it back hot. Mash the brake , let it up, go around and give the wheels a spin. See who's dragging. See which rotor shows bluing. Check your wheel bearings for excess play which would contribute to heat buildup. I would do these things with all other mentioned "could-be" s . Good luck
 
I almost forgot. Do you have power brakes? Your booster diaphram could be migrating in the can applying the brakes with the foot pedal pushrod fully released. Something to look into.
 
inertia said:
I'm thinking if the brakes release by cracking the line at the M/C,, that a rubber hose will only hold pressure in an localized area,,

I think the M/C rods outta adjustment, if you changed M/C.. or if it's the original Master cyl, it still has a residual pressure valve in the front system (re Dubob).., or the M/c cups have swollen..

hope it helps
Click to expand...

I can check that sometime this week.

The rubber hoses are new and so was the master cylinder. I also have a brake booster.

Thanks for the ideas everybody.
 
There is a port timing hole in the fluid reservoir. The primary seal on the front of the piston in the master cylinder must return past this port when the pedal is released. This will release any pressure in the system due to fluid expansion when the fluid gets hot. It is usually a very small hole about .022 in. in diameter. Look to see that its there, it's not plugged, and you can push fluid past it by pushing on a brake caliper piston.
 
I tried adjusting the pushrod coming out of the booster so it would be shorter. This seemed to help some. When I was taking the master cylinder on and off the mounting studs I noticed the piston is actually compressing when I put the master cylinder on (I can see the fluid swirl). When I have everything put back together with the cap off, I press the brake pedal and don't see the fluid swirl in the bowls.

Currently I have the pushrod adjusted as far back as I can. Would I need to space the master cylinder from the booster at this point? If I do, why would this adjustment not be set properly from the get go because the manufacturer asks for a 1/64" of air gap from the piston and pushrod; that seems hard to accomplish.
 
Did you change your prop valve when you installed the disc?
 
Jacmifish said:
When I was taking the master cylinder on and off the mounting studs I noticed the piston is actually compressing when I put the master cylinder on (I can see the fluid swirl).
Click to expand...

THIS IS YOUR PROBLEM. The master piston is not returning far enough. The piston MUST return ALL the way back

You may have

wrong / incompatible master

Something wrong with booster, assembled wrong

Problems under the dash with that end, linkage, return, pushrod, etc.
 
I didn't change the proportioning valve. I have an adjustable valve I'm not using but I thought that would be only for the rear drums pressure adjustment?

67DART273, I plan to go buy some thick gasket material so I can offset the master cylinder. After I find the offset distance I need from the gaskets I can to plan to buy or make a metal offset plate.
 
Jacmifish said:
I didn't change the proportioning valve. I have an adjustable valve I'm not using but I thought that would be only for the rear drums pressure adjustment?
Click to expand...

The proportioning valve you have should be fine, you would need to change it if you go to disc on the rear.
 
I spaced the master cylinder off the booster by a nut on each stud. It seemed to be a little too much but it made the brakes act completely normal now.

Did I miss some other adjustment to make this happen or is this something people have to do? I guess my question now is if the push rod can be adjusted from the firewall side or will I need to stick with the spacer?
 
There is an adjustable pushrod if I'm not mistaken.
 
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