Brakes Hydraboost Swap

Ed Blount · Jul 15, 2013

I have just finished a Hydraboost swap on my '92 5.0 LX using a '96 GT system and I have a small dilemma. Everything went great with fitment and the system works great on the shop floor but when I did a road test the front brakes hold some residual pressure cause wheel lock (RATS).
I have worked my way back from the calipers to the distribution valve replacing parts as I went
with no luck. The master cylinder is new and I even went so far as to change the boost
pump, still I have the same problem. Does any one have some inside information or suggestions that might help me? I have thought about shimming the MC out from the booster to see if that helps.?.

TOOLOW91 · Jul 15, 2013

maybe your thoughts are right and the mc rod is staying partly engaged.

jrichker · Jul 15, 2013

On vacuum power brakes, the booster to master cylinder pushrod is adjustable. Check to see if your setup has a similar adjustment. see http://home.comcast.net/~mjbobbitt/mustang/page1.html for more Mustang brake help.

Ed Blount · Jul 15, 2013

jrichker said:
On vacuum power brakes, the booster to master cylinder pushrod is adjustable. Check to see if your setup has a similar adjustment. see http://home.comcast.net/~mjbobbitt/mustang/page1.html for more Mustang brake help.

Ed Blount · Jul 15, 2013

Push rod on Hydraboost is non-adjustable.

jrichker · Jul 16, 2013

Ed Blount said:
Push rod on Hydraboost is non-adjustable.

Maybe time to get creative and fabricate your own...

Do both front brakes tend to drag or lock, or is it just one side?

Ed Blount · Jul 17, 2013

jrichker said:
Maybe time to get creative and fabricate your own...

Do both front brakes tend to drag or lock, or is it just one side?

Both wheels drag. I let it sit for a day and the pressure went down. I'm tinking about putting a higher tension spring in the MC to push it back. I have a couple of old Fox MC's laying around to work with so it would be a cash free experiment.
I think I failed to say earlier that I tried bleeding back the booster but that didn't work either. Bum-r.

2000xp8 · Jul 17, 2013

While I don't know what your issue is, I do know I used a 96-98 hydroboost without modification. So I would think that changing the spring is not the solution.

jrichker · Jul 17, 2013

Are you reusing the old front brake lines? I have heard that old line will sometimes develop an internal leak that allows fluid to seep in between the layers of the hose. The result is that it acts as a one way check valve. The pressure on the outer layer is greater than the pressure inside the hose, pinching the inner hose closed.

Ed Blount · Jul 17, 2013

2000xp8 said:
While I don't know what your issue is, I do know I used a 96-98 hydroboost without modification. So I would think that changing the spring is not the solution.

Yes it was supposed to be a week end project out with the old and in with the new have a hose or two made up and be on my way.

Ed Blount · Jul 17, 2013

jrichker said:
Are you reusing the old front brake lines? I have heard that old line will sometimes develop an internal leak that allows fluid to seep in between the layers of the hose. The result is that it acts as a one way check valve. The pressure on the outer layer is greater than the pressure inside the hose, pinching the inner hose closed.

Thats an interesting thought but what are the odds of it happening on both lines at the same time, still its worth looking at.

Ed Blount · Jul 17, 2013

Ed Blount said:
Yes it was supposed to be a week end project out with the old and in with the new have a hose or two made up and be on my way.

How much pressure did you have to apply to seat the MC onto the booster? I read something about 5Lb is required.

2000xp8 · Jul 18, 2013

Ed Blount said:
How much pressure did you have to apply to seat the MC onto the booster? I read something about 5Lb is required.

Ed, may want to work on your quoting and proofreading a bit, probably not going to get too many replies when quoting yourself, lol.

If you were asking me, i never took the mc off the booster.

What is your complete brake setup all the way around? Did you gut the Proportioning valve? Rear discs? Stock front brakes?

Mustang5L5 · Jul 18, 2013

When the front brakes drag, loosen the MC on the hydroboost setup and see if that frees them up. That would tell you if the pushrod is still engaged to the MC piston

Ed Blount · Jul 18, 2013

2000xp8 said:
Ed, may want to work on your quoting and proofreading a bit, probably not going to get too many replies when quoting yourself, lol.

If you were asking me, i never took the mc off the booster.

What is your complete brake setup all the way around? Did you gut the Proportioning valve? Rear discs? Stock front brakes?

2000xp8
I thank you for your constructive criticism. I'll work on it but I'm not sure where I did the quoting perhaps you can point it out to me.
I'm new at this so go easy on me.

The brake set-up is four wheel disc from SSB with BigBite rotors all around and no I have not gutted the proportioning valve. I did on the original valve per the instructions of SSB but I didn't think it was necessary on the valve that came on the hyrabooster and master cylinder.
Thanks , Ed

Ed Blount · Jul 18, 2013

Mustang5L5 said:
When the front brakes drag, loosen the MC on the hydroboost setup and see if that frees them up. That would tell you if the pushrod is still engaged to the MC piston

Yes, that's first on my list.

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