Ford Mustang Hydro Boost and ATE master cylinder Rod Interference

Gas prices are high, and I have my 4 cylinder working, couldn’t ask for better time to knock out some repairs and improvements on the 944. First issue for now, my dragging brakes and hard brake pedal. I’ll start with some history. My breaks were always hard to bleed. I seemed to have trouble cycling brake fluid through the system. I do have new lines/hoses and new Wilwood 4 pots on all corners. That was the first sign that I ignored that something was wrong. I went ahead and replaced the master cylinder with a new ATE unit. Things seemed to improve slightly. I drove the car mostly on short trips. After a while, I noticed the brakes would stiffen up and drag in stop and go traffic. My pedal was becoming rock hard after a 20 minutes of driving. After some research I suspected my brake booster was preventing my brake master cylinder plunger form fully returning. My fluid was heating up and not allowed to return to the reservoir, pressurizing my brake system. I loosened the master cylinder and noticed I had about .18” of interference between the HB rod and the brake master cylinder. I confirmed this was the problem by installing some washers between the HB adapter and the master cylinder. I can confirm ZERO brake issues now. At first I thought was my adapter was not thick enough, however after an inspection, it looks like my HB rod is not fully seating in the ATE brake master cylinder. I painted my HB rod with a paint pen and noticed it only engages a small ring about .2” from the bottom cone of the master cylinder. I measured the engagement with a master cylinder/ booster measurement tool and noticed I should have .018 clearance, right on spec. The rod on the measurement tool is of course smaller diameter than the HB rod, so I suspect its fully seating into the master cylinder. So has anyone had this issue with the ford rod not fully seating into the brake master cylinder, if so how was this resolved? I may just get another master cylinder if that solves these issues, or maybe remove the rod from the HB somehow and taper the end. I’m surprised there aren’t other posts about this.

How are you running the return lines on the hydroboost? Are they teed together or run separately to the reservoir?

Caindog, I suggest you contact Kent directly. No HB on my car, but I thought TPC had the nut fully cracked on that one.

I did have my return lines teed before my microfiber filter. The HB return is now teed after the filter, directly before the power steering return tank. I didn’t notice much a difference with or without the added return line back pressure from the filter. I will reach out to Kent and post back.

Due to manufacturing tolerances, sometimes you have to grind the rod a little to insure no preload on the master cylinder. About 25% of the time, I have had to grind to fit.

I took everything apart and figured out the interference. I noticed I still had preload with my hydro boost rod removed. It looks like my master cylinder is interfering with my hydro boost spring retainer. I took off about 1/4 inch from the master cylinder and now I have no preload. For reference my hydro boost is from a 1996-2004 mustang and my master cylinder is ATE.


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Interesting. First time i've seen that happen. Good find

Hi , I have just got mine running for the first time today and as described above when the motor is running I have quite a bit of brake drag . It appears Cain has 1/2 an inch of the rod that goes into the brake master cylinder , is this correct ? Any help would be awesome, thanks Brett

There is quite a bit of rod that goes into the master cylinder, I'm not sure why the master cylinder has such a deep cavity. I did not take a measurement and now the system is back together. When installing the hydroboost using the 944 master cylinder, or any master cylinder for that matter, I would check all the interfaces. It doesn't take long to do, its a pain to constantly bleed your brakes or have power steering fluid dipping all over you. Im sure others will chime in with their recommendations as well but this has worked for me.

(1) Verify zero preload when installing the master cylinder to hydroboost. Some seem some have gotten away with very small preloads but over .125" that I had will not work. If you have preload take the rod out of the hybroboost and verify the master cylinder plunger is not interfering. If you have a brake booster push rod measuring tool, might be a good time to verify your pushrod clearance .020" seems to be preferred. With the TPC adapter I had .018 clearance so I did not have to shorten my pushrod.

(2) Ensure pedal side of hydroboost retracts all the way back. I threaded the end of my hydroboost and installed a jam nut on the rod to clevis. I set my pedal position, ensured I had full retraction, and tightened the jam nut.

(3) There should be minimal backpressure on the hydro boost return line. I do run a microfine filter in my power steering system but it only filters the return from the power steering rack.

If your still having issues, you may have a mechanical failure. My main symptoms were a hard pedal after city driving, hard to bleed the brakes, esp the front, and the brakes would release after the car cooled or I loosened the master cylinder bolts and allowed the master cylinder to move forward ~.2 inches. Usually failed master cylinders creep forward if you hold them down, you will feel the pedal slowly drop to the floor. I'm unsure on typical hydro boosts failures since I have never failed one on any of my cars with hydroboosts. Make sure your calipers and emergency brake shoes are not dragging. Also check your brake lines/hoses for damage, unlikely to be a line but worth checking, its not fun rupturing a brake line when your on the road. That's all I can think of regarding brake drag. It could also be a bad wheel bearing but it would have to be very bad, easily noticeable, for it to be enough to cause noticeable brake drag.

Thank you so much for your efforts, I will go through all you have said tomorrow and see if I can work it out

Forgot one last item.
The system needs to be bled. There are some tedious procedures that the truck guys do with hydroboost, i.e. turning the wheel left/right 5 times and press brakes 3 times, repeat 40 times. I have never done that, maybe I should, who knows, I always seem to have some residual air after I crack open the system. I just typically move the wheel from lock to lock a few times, press the brakes hard a few times, crack the hydroboost bleed nipple, and free rev the car close to redline a few times. That seems to work for me. I'm guessing the residual air expands when it gets hot and keeps some slight pressure on the brakes, also the steering feels off. Hopefully you get your system working properly.