The famous Hydroboost issues....

I need help!!! I have a pretty much completed and ready to test drive LS1 swapped 951, but can’t cuz we have no brakes...
Brand new 951 master, got the mustang hydro kit including pump all from Kent at TPC. Installed everything, drilled pedal 3/4” higher. Bled brakes, bled PS system. Pedal goes to floor and stays there, have some rear brake action but less in the front. I can still turn the wheels with my hand when pedal is to the floor, I can feel friction. Any ideas what this could be??? Been at it for about 2-3 weeks and have 0 clue what could be the issue....

Is it doing this with the engine running or off? If it's doing it while running, what happens to the brake pedal with the engine off? I have seen a hydroboost do this when the lines were hooked up backwards. While engine running, the pressure would try to pull the pedal down and not come back up. I have also seen a defective HB cause the same.
If you pedal is firm and the brakes work with the engine off, I would recheck your lines. If they are hooked up correctly, then I would say the HB may be bad.

Yeah it does same with Engine on or off makes no difference..

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You may have a bad HB then. It may be sticking internally or something.

One other possibility is that the pushrod between the master cylinder and the HB could be too long. That will cause the master cylinder to not fill completely. I had that happen once. Pedal was very low and brakes barely worked.

Hotrodz of Dallas wrote:You may have a bad HB then. It may be sticking internally or something.

One other possibility is that the pushrod between the master cylinder and the HB could be too long. That will cause the master cylinder to not fill completely. I had that happen once. Pedal was very low and brakes barely worked.

Interesting, do you know how long the pushrod should be? Or how long yours was? And did you buy a different pushrod or did u just modify the existing one?

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I don't remember the length. But, it needs to just touch the master cylinder piston when install on the HB.

If i'm not mistaken, Kent has pushrods made for that. If not, he may know the length.

Hotrodz of Dallas wrote:I don't remember the length. But, it needs to just touch the master cylinder piston when install on the HB.

If i'm not mistaken, Kent has pushrods made for that. If not, he may know the length.

Yeah my pushrod came from Kent.
But it pushes the mc about 1/2”-3/4” when it touches the hb

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if the piston of the master cylinder is being compressed when your bolting it on, thats your problem. on any braking system if the master cylinder piston is compressed at all in its "resting" state brake fluid wont move from the reservoir to the cylinder. the pushrod on the end of the booster should actually have a very slight gap between it and the master cylinder piston.

sharkey wrote:if the piston of the master cylinder is being compressed when your bolting it on, thats your problem. on any braking system if the master cylinder piston is compressed at all in its "resting" state brake fluid wont move from the reservoir to the cylinder. the pushrod on the end of the booster should actually have a very slight gap between it and the master cylinder piston.

Ok perfect thanks guys for the help! Now can I modify my pushrod to be shorter? Or should I buy a different one?

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Ok so I was able to talk to Kent and played around with the hydro some, he told me to remove the master, remove the low pressure line, remove the Clevis from the pedal and pull the clevis as much as I could towards the rear of the car, if done correctly I should get a squirt out the hydro where the low pressure line was removed. And after all that if the master went onto the hydro properly without any master piston compression (which it successfully did! Thanks Kent!) to move onto part two, which was firing the car up and steering all the way to left and right and holding it at bump steer for about 2 seconds and repeat this step 10 times each side. After doing all this the brakes seemed to have gotten better! The pedal is returning now (but slowly), and the pedal does feel pretty soft sadly and at full pedal compression I can still turn the wheels, it’s very hard but I can still turn them with just my hands...so I went further and attempted to bleed the calipers again and didn’t get any bubbles out of them, just solid streams of brake fluid. Now a friend of mine did tell me that it sounded like I may have some air in my master so I went ahead and pulled the master and bench bled it again, (the master is new since I started the Swap), I Bench bled and hooked the lines back up to the car, bled the calipers again, once again no air bubbles out of the calipers. Now my question is.....how can I be so sure that I have a good master? Cuz I wouldn’t be surprised if I got a defective one, or damaged this one internally somehow. I am wondering is it possible to compress the master piston fully, by pushing the master by hand against a hard object such as the edge of the hydro/booster, or firewall? Or should that just be impossible by hand to fully compress it?

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Well.....so we were about to go through the whole bleeding process of the calipers for the millionth time, when...(keep in mind he bought this 86 951 specifically to LS swap it, neither of us have ever touched or worked on a 951 before this one..) he was attaching the bleeder hose on the bleeder screw, he looked up and behind the caliper and noticed a SECOND bleeder screw.....long story short...we lept with joy and giggled like little girls and bled all the calipers again through BOTH bleeder screws on EACH caliper and voila we have brakes and test drove the car. My god we are in love with this work of art!! Drove perfect! Thanks for all the help guys! Really appreciate it! Couldn’t of done it without you guys!

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Sometimes the simplest things give us the most joy. Congrats on finding the issue.