Brake Pressure Issue-Post Brake Swap

[Axes-Of-Evil]

Hi guys, I ran into a bit of a unique situation (I think) when I switched out my brakes.

So, not entirely sure if it will help but here is the (extensive) full back story:

I was getting prepared for my first time going out on an open track (Streets of Willow Springs) and had stock brakes, pads, and fluid. I knew I needed different ones for the track so I ordered some carbotech xp10 and xp8's along with brand new rotors, I also picked up 3 bottles of Motul RBF600. The car also used to be a v6 so I also ordered new caliper brackets to fit the new GT sized front rotors. I lifted the car up, took off the old rotors, pads, and caliper brackets then proceeded to put on the new ones with compressing the rear piston for the ebrake. We then bled the brakes. We started at the back right, then back left, front right, then front left and kept cycling around them until we saw nothing but clean fluid coming out of the brakes. We used up all 3 bottles to make sure it was a good solid flush. During the bleeding process we didn't see any air bubbles coming out of the lines and we would open the zerks as I pressed on the pedal, then close them before I released it to ensure no air got back into the lines. If it did in that process I would presume I would see bubbles coming out on the subsequent pushes but didn't see any, just some really dirty fluid at the start of the process. Also, we made sure the reservoir stayed above the mid line and never sucked air in that way.

After putting everything back together etc. We turned the car on and the pedal went to the floor (I expected that because we compressed all the brakes and obviously they need to get back to where they were pushing up against the rotors at a regular stroke. However, this is when we noticed the problem. After doing a few pumps the pedal gave the feeling that it's had since. It gives a little bit of feedback and bite in the first inch of travel, then it just goes dead and pretty much goes all the way down to the bottom until the last two or three inches and it starts to give some pressure again. It's really weird. I expected less feed back with track brakes but not a dead pedal for most of the travel. When the car is off, after a couple of pumps it goes back to a normal pressure feel but when teh car is turned on it gives the weird travel with little to no feedback. It's also been like this for two weeks now (also a DD) so any air in the lines should have at least traveled around. I had a buddy drive it who has tracked his mustang and done multiple brake swaps and he said it sounds like the brake booster isn't working when the car is on. Any ideas?

TLDR:
Did a brake swap, 99% positive no air in the lines, pedal feels fine after one or two pumps when the car is off but when the car is on there's feedback for the first inch or two then it goes dead until the last inch or two down on the floor. Friend said it sounds like a potentially dead brake booster, but I don't know why the brake booster would just die out of nowhere. Brakes were fully functional with no sign of issues prior to the swap. Suggestions or experience with this issue?

 

[SoundGuyDave]

I had a similar problem on my SN95 race car, when one of the piston seals on the master let go, but it only showed up when hot...

First off, let's eliminate the obvious stuff. Pull the vac line to the booster, and plug it, and then (gingerly!) drive the car and see how the pedal feel is compared to what you have now. If ALL your issues go away, swap the booster out and you should be good. It sounds like the valve in the booster may have gotten gefucht somewhere during the bleeding process. If the vac bypass doesn't do anything for you, reconnect it, then unplug the ABS harness from the HCU and see if THAT does anything. It shouldn't. If it DOES, then I'll bet 99% that you have something weird going on with the brake package you put on the car. Double-check that the inboard rear pads have the lineup pins in the notches on the piston , and that there's nothing funky going on there. Pull the front calipers off and eyeball those as well. Then, re-bleed the brakes... This time, use a hose leading into an empty water bottle with a couple of Oz of brake fluid in it, making sure the hose is below the level of the fluid in the bottle. That way it's impossible for air to be drawn back into the braking system once you have a solid column of fluid from the bleeder to the bottle. Work out a plan with your assistant (may have done so, but I'm troubleshooting from the rust-belt, so...): With the bleeder closed, he says "Pump!" and after a couple-three strokes on the pedal, put your foot down and call "Holding!" That's his cue to crack open the bleeder. As soon as your foot hits the floor, call "Down!" and he'll close the bleeder. Repeat as necessary. No need to flush, this is just to double-check that the hydraulics all appear to be functioning; bleeder flow-rates are all about the same (on each axle, rears will have a LOT less fluid volume per stroke), etc.

Also, fairly rare, but it does happen, there can be a minute amount of corrosion in the far reaches of the master cylinder bore, where the piston rod and cup seals NEVER get to with the hydraulics closed. During bleeding, with the pedal going all the way down, you can run the seal across that rough area and scuff the seal, leading to high-pressure-only fluid bypass. In other words, with the booster dumped, you can't generate enough line pressure with short power-brake pedal ratio, but soon as the booster comes alive you get enough line pressure to push past the seal... As I said, fairly rare, but it happens.

That's all I can think of off the top of my head... Just think through how the hydraulics work, and you can see that if you're getting a pedal drop, that means that either something in the hardware end is MOVING (booster, caliper mounts, pads, bearings), and quite a bit, or the fluid is getting bypassed around the circuit, somehow. The only two places that can happen are in the master cylinder and the ABS HCU, hence my advice to check them first...

If you figure out what it is, let us know! This is a weird one... Oh, like a light bulb, boosters are one of those things that either work, or they don't. If the booster died (diaphragm or valve failed) it's most likely nothing you did, it was just it's time to fail...

 

[NUTCASE]

Some good points by sound guy

Defunatly make sure the notches in your rear calipers piston are at 12 o'clock. This is a common error.

If still trouble get a vaccume pump on the resevior similar to the process of bleeding the clutch. It sounds wierd but it works.

When I got my car I always thought the brake was spongy. Did a line lock and the the brakes got more spongy no matter how much I bled it. Then I did ss lines and no matter how much I bled it it was still spongy

Then when I did my clutch I used the vaccume pump on the resevior method. 2nd iteration I got bubbles coming up so hard the splash kicked my stopper off. After that not only did my clutch work, but my brakes were better than the day I drove my car home

 

[Axes-Of-Evil]

I'm pretty positive the notches were lined up properly, I won't have time to pull the wheels and look at it this weekend but could just that really cause that big of an issue in pedal feel? Ill definitely try some of those suggestions. Especially pulling the hose and reconnecting it first since I don't have a vacuum and will have to ask a friend for that one. I don't have ABS if that simplifies it. Should have mentioned it. Thanks for the help guys. Ill keep you posted through the week as I try the different fixes.

 

[SoundGuyDave]

Rear pad seating: There is an amount of play in there, where if the pads are not completely seated in the notch, you can get some funky results, like the caliper rocking slightly around the slide pins. Not high-probability, but well worth checking, particularly as it only takes a few minutes...

ABS: Without that pump in-circuit, it really does simplify things... From what you're saying, it sounds like either you have play in the mechanical side, or a hydraulic issue in the master. Fairly easy to sort that out, with the car up on jackstands and the wheels off.

Go for the booster first. That whole test will take MAYBE two minutes, and the only tools required are a pair of pliers (vac line clamp at the master) and a stick (to plug the hose). At least that'll settle the booster issue!

 

[DRock]

Bleed them again to just make sure?

 

[steveespo]

I had a funny pedal feel at VIR during my first 2 sessions Memorial day weekend. Not a soft pedal but not firm as you would expect with fresh fluid and a bleed. My buddy suggested engaging the ABS in the paddock a few times so I did. Went out and did next session and brakes felt much better. We then swapped pads on the rear and re bled and found a few air bubbles in both rear calipers. Rock solid after that. Aren't there code scanners that you can plug in to activate the ABS pump circuit? If anyone has one that does please chime in.
Steve

 

[Vorshlag-Fair]

I tracked a 2013 GT once on the stock 13.2" rotors, stock pads and stock fluid (mistake!). Burned up the fluid in 2 sessions, took the pads to the backing plates, and drove off the track at 80+ mph when the pedal went to mush.

We later flushed out the OEM fluid (made from recycled donkey piss) and put in RBF600 fluid, new Carbotech pads, etc. Never could get the pedal to come back. Bled the car like 4 more times, still had a mushy pedal. WTF? Turns out we melted some seals in the master cylinder... after we swapped in a new replacement MC the pedal is rock hard.

Probably not your issue, but its something to look for when you have a weird pedal and you can't get it to bleed. We've also had two brake vacuum booster check valves fail, and that makes ALL vacuum assist go away (rock hard pedal with no assist). Gotta love Mustangs...

 

[steveespo]

Thanks Terry,
I suspected the master cylinder seals too and plan on swapping it out this fall/winter. Cost is ridiculously low, $84 for all new MC and reservoir including a new cap, seals, gasket, screw and fluid level switch.

As far as the check valve in the booster is that a servicable part or do you have to change the booster?
Steve

 

[Vorshlag-Fair]

Thanks Terry,
I suspected the master cylinder seals too and plan on swapping it out this fall/winter. Cost is ridiculously low, $84 for all new MC and reservoir including a new cap, seals, gasket, screw and fluid level switch.

As far as the check valve in the booster is that a servicable part or do you have to change the booster?
Steve

Yea, the MC was thankfully very inexpensive and easy to swap. Worked like a charm.

The check valve is like a $15 part and takes 45 seconds to change, from the outside. See part number below. Every S197 race shop should keep one of these as spares in your shop and race trailers - we've had TWO of these fail now.

This check valve first failed during the June 2012 Optima Challenge event on our car. I was fighting a ROCK hard brake pedal and ZERO assist during their speed-stop event, autocross and track events. I managed to "drive around the issue" in the autocross (1st place), track event (3rd place) but the speed-stop killed my chances at an overall win (14th!). We replaced this check valve on the whim of my head tech the next week and fixed ALL of the vacuum assist problems. It wasn't visibly damaged and didn't leak vacuum externally, just wasn't holding the vacuum in during long WOT portions of racing. Didn't appear on the road at all. Maddening! I hate it when a simple little $15 part ruins your chances at an event like this, that was televised.

The "shop rag zip tied around the brake reservoir cap" is an old trick that saves your underhood paint from the small amount of fluid that can dribble out when it gets VERY hot. And on these big cars, the fluid gets VERY hot. Pretty obvious to most, just wanted to point it out in case people ask. We do this on most brake MC's and even on power steering pump reservoir caps for some GM cars, too.

The Mustang's brakes worked flawlessly this time out, so that was a big plus! During the two weeks since Optima we replaced the original vacuum check-valve at the brake booster, which wasn't expensive. We weren't sure if the power assist would work until we revved the piss out of it on track. Well, once on track it was obvious - that fixed it! No telling how long this valve has been bad or "going bad", and leaking down vacuum, but hopefully this is the end of our "brick-like brake pedal" after long acceleration runs on track and in autocrosses. Keep this in mind if you run into similar a lack of brake assist.

The only other problem we had with the "vacuum assist brakes", or leaking vacuum in this case, on this car in 3 years was a broken plastic fitting on the JLT oil catch can kit. This allowed all of the vacuum to dump and we lost power assist again. The lines and fittings in their kit are junk, but for the price I can see why. The catch can itself is OK, if a tad small. We changed all of the lines and fittings to real metal fittings and lines and made a bracket to mount the catch can, so it doesn't hang by the lines. Haven't had another vacuum issue since.

Cheers,

 

[Axes-Of-Evil]

I opened up the reservoir this morning and noticed all of the brand new brake fluid looks really murky and dirty so it's most likely a seal broke somewhere but to save time and get my far up and running ASAP I think I'm just going to replace it all from the reservoir to the booster. Where did you guys purchase the assembly for an inexpensive amount? I know I can get a lot of it from buyfordparts.com but just want to know if there's somewhere better to go. Thanks for all the help guys.

 

[SoundGuyDave]

Looks like what you're looking for is the complete master cylinder assembly... The usual suspects should have it. While you're there, swap the booster check valve; Terry thoughtfully included the part number in his first post...

 

[steveespo]

Tousely or Levittown Ford have great prices on Ford service parts and shipping.
Steve

 

[Axes-Of-Evil]

Okay, so jumped the gun on evaluating the brake fluid before. Should have waited until I had better lighting and more time. Re-checked it this weekend when swapping the pads/rotors back to street and the fluid looked fine actually. Also had time to try the booster bypass trick and when I did it the pedal went hard as a rock. I ordered the replacement part that Terry listed and it will arrive tomorrow. Hopefully that's all it is and I can report back in a day or two that the check valve was the only issue.

 

[SoundGuyDave]

Awesome!! It would be fantastic if it was only a $20/5-minute fix!

 

[Vorshlag-Fair]

Okay, so jumped the gun on evaluating the brake fluid before. Should have waited until I had better lighting and more time. Re-checked it this weekend when swapping the pads/rotors back to street and the fluid looked fine actually. Also had time to try the booster bypass trick and when I did it the pedal went hard as a rock. I ordered the replacement part that Terry listed and it will arrive tomorrow. Hopefully that's all it is and I can report back in a day or two that the check valve was the only issue.

Yep, figured that might be worth a try. Like I said, this check valve failure has happened to us twice.

 

[Axes-Of-Evil]

Okay I swapped the valve out an the pedal feels a bit more firm. But it does feel like it might be a bit softer than it should be but I'm wondering now if I just forget how the stock pedal feels because I've had it be SO dead for a couple weeks that now the potentially kind of dead feels like stock. Basically just wondering if I'm expecting it to be much more firm than it should be..

 

[steveespo]

I decided to do a complete hydraulic circuit overhaul on my car. Just got the new master cylinder and reservoir today, new booster expected Tuesday. Will swap and report on feel, performance changes after swap.
Does anyone know the brake line sizes for the master cylinder? I want to buy some fittings and tubing to create bench bleeding pipes.
Thanks
Steve

 

[Axes-Of-Evil]

Thought I'd come back and give a much delayed update. My rear right caliper had failed. With the parking break on, the rotor had about an inch of play to both sides. Swapped out both of the backs just in case the other was on its way as well and the brake pressure went back to normal and just in time for me to have gone out to Buttonwillow for the first time!