Partial Donor Build - Brake Issues Still.. What am I missing ?

[Racee75]

I am currently building a MkIV starting with the base kit and the brake system came from a 1997 Mustang GT.

I have everything installed in the car but I am unable to get a "good feel" to the brake pedal. I have bleed all the line multiple times ( at least 2 pints of brake fluid ), no visible fluid leaks, ABS system passes self test and pump and solenoids are operable.

I changed the master cylinder last weekend, re-bled and good hard pedal but then found I had front brakes dragging that I could resolve by opening bleed screw and releasing pressure. I then replaced flex hoses incase they were the issue.. no difference.

I then started to remove the new master cylinder and as I pulled the mater cylinder away from the hydroboost unit the pressure was relived from the front brakes. I made a spacer to put between the master cylinder and booster to test to see if the brakes would still drag. They do not but I am back to a not so great pedal.

Any ideas on what I am missing or doing wrong ? Starting to really get frustrated by this.

Thanks in Advance.

[Gumball]

With ABS and power boost, your braking system if more complicated than mine, but just one suggestion - try an adjustable rod from the pedal to the M/C... it may be that you're not releasing the brakes fully when the pedal is in the up position.

[edwardb]

Define "dragging." It's normal with disk brakes for the pads to have some pre-load against the rotor when the pedal is released. Wheels should still turn, but you will hear kind of a scraping sound of the pads against the rotor. You want them that way.

[Racee75]

Ebwardb.... as far as the amount of dragging there is a significant increase of pressure needed to turn the wheels by hand compared to when the calipers have been"bled off". There is the normal pad scrap when the wheels turn without the pressure build up.

[steno]

Paul is correct about the drag on a released disc brake. I was thinking the same as Chris. That worked for me!!

[phileas_fogg]

...It's normal with disk brakes for the pads to have some pre-load against the rotor when the pedal is released. Wheels should still turn, but you will hear kind of a scraping sound of the pads against the rotor. You want them that way.

That's some great information right there, and alleviates a concern I had about my Wilwood installation. Thanks Paul!


John

[Jeff Kleiner]

Light drag is normal but residual hydraulic pressure is not. You need to assure that the master cylinder pushrod is fully disengaged when the pedal is released. Sounds like you were on the right track when you added shims---check to be sure that the pedal is not contacting the tube that runs side to side below the steering shaft which would prevent it from fully releasing pressure at the master. One last thing regarding the "not so great" pedal feel---if you happen to get the calipers swapped to the wrong side they will appear to bleed well but a pocket of air can remain at the top of the chamber which will cause a soft pedal.

Good luck and keep us updated!

Jeff

[Racee75]

Thanks All for the advice, I think I may have found the issue with the residual hydraulic pressure. I checked the pedal side of the booster and the pushrod seems to disengage fully. However on the master cylinder side once i fully undid the master cylinder I can feel the master cylinder plunger touch the pushrod about 1/8" before the mating surface. Next weekend I will remove the mater cylinder and check the depth of the pushrod recess.

One other thing is as I am not familiar with the Hydroboost system operation, is it possible the the output pushrod may retract once hydraulic pressure is applied to the booster ?

[CraigS]

I haven't worked on hydroboost but have worked on plenty of vac boosters so hopefully my thoughts will be verified (or blown away) by someone w/ more hydroboost experience. As you realize the pedal pushrod needs to have a tiny bit of freeplay so the system releases completely. Also, there is a pushrod in the hydroboost unit which needs a tiny bit of play between it and the MC. You noticed that here;
"However on the master cylinder side once i fully undid the master cylinder I can feel the master cylinder plunger touch the pushrod about 1/8" before the mating surface."
On vac boosters the pushrod in the booster is usually adjustable for length. It can get complicated to measure but what you want is something like .025 inch of freeplay whan the MC is bolted tight. I hope the pushrod in your hydroboost is also adjustable as you could shim it but that gets more difficult to get the right clearance. As you might suspect, too much clearance uses up pedal travel. Good luck w/ it.