Braking woes with S550pp conversion and GT500 booster

[captdistraction]

I'm tracking down a braking issue on my car that's sidelining my racing program.

The setup:
2011 GT racecar:
S197master cylinder, 13-14GT500 vacuum booster
new vacuum line to CJ manifold
new vacuum valve
stock lines from master to ABS and chassis hardlines, stock ABS valvebody
Race Abs controller
blowfish master cylinder brace (which was rubbing one of the lines, but didn't appear to crimp it)
S550 PP brake calipers, 13-14GT500 rotors, centering spacers
Gloc R18 pads up front, 13-14 GT500 brakes out back with DTC60 pads
Stainless lines

This setup previously worked fine with the stock GT booster, but had a softish pedal that didn't have much bite until low, but would claw one's face off when asked. This last weekend I ran a faster track, but had 2 issues:

1) variable boost from the booster, sometimes it was there, sometimes it wasn't, sometimes it worked for the first 1.5" of travel, then would be exhausted or repeated pressings would work for first press but then second press felt mostly manual

2) there was no bite at the top of the pedal, had to leg press the hell out of the system to generate braking torque.

This led to a very serious off during practice that nearly put the car in a wall at a alarming speed, and during the race I couldn't push more than 7/10's, and nearly punted a few cars as I had to check what sort of brakes I had going into each corner.

I've checked the system, had my ducts blocked off to ensure enough heat for pad bite was there, the calipers did reach 370* (not sure on rotor temps but looked nominal) and if I stood on it hard enough I could get the car to slow way down. However it took a leg press that was impossible to drive competitively with. My hamstring is still sore on the right side.

I've bled $200 worth of fluid through the system, and at this point completely lost. I believe it to be a hydraulic or vacuum issue, perhaps the wrong booster since the old booster worked better (we had tried restricting it, but that made matters worse as the pedal was a rock after what little boost there was). I'm thinking somewhere there's a restriction but not sure what. I've pulled everything apart and checked it, hooked up every diagnostic I can. Everything "looks" great.

before I start blanket replacing parts, any thoughts?

I have been looking at the Kohr master kit/booster and hardline to abs, thought about replacing the abs valvebody but it seemed to work fine when we put a IDS on it to do a bleed procedure, no errors in the system outside what is expected with the abs race box, all wheel sensors ok, etc. I don't think ABS is taking away braking. Also thought about putting a S550 brake booster and master on (and fabricating the hard lines) since those are a match for the front calipers (and the rears are similar)

here's a picture showing the car getting on its nose (note my head up as I'm sure I'm thinking about what questions to ask as I meet my maker):

 

[venom52]

To me it sounds like there's a issue some where. We know that others have the same setup and haven't reported any issues such as what you have currently. I know when we switched the wife's car over 6 pot fronts and just bled them a couple of times. There's been no issues with her brakes at all. Her's auto even with a CJ intake etc. It's almost as though the system is sucking air from some where giving you the soft pedal.

 

[RES_22]

Excellent level of detail...

Thoughts:

1. Vacuum valve issue...hairline crack that could be heat or vibration (fatigue) sensitive?
2. Vacuum line issue...similar failure mode as "1" above. I am sure that you've ziptied the $hit out of the thing...so I'm just thinking out loud.
3. If those check out...then the odds are it is a booster issue...the only significant delta between what worked consistently vs a configuration that tried to bring you closer to Je$u$.

At the end of the day it sure sounds like a vacuum problem.

************

Could be a fun secondary discussion to have with Kohr if you call about their m'cyl/booster kit.

 

[Grant 302]

I see two potential problems that would create no-assist conditions. Vacuum not returning to that back side of the diaphragm when when pedal is released. Or ambient not entering back of diaphragm when pedal is depressed.

Based on your description, I suspect the former. I'm thinking either lack of pedal return (possibly from mis-matched parts) or loss of vacuum perhaps due to bad check valve or a seal not properly opening or closing due to mis-matched parts and expected travel to make seals open and close properly. Intermittent loss of vacuum from the engine could also provide inconsistent braking. If vacuum lost there while vacuum is being held by the check valve, it will be good for one more assist, but fail to at the next pedal press.

 

[Swiss Boss]

Could it have been pads knock back you experienced with your stock GT booster?

How's the seal between the master and the booster?

 

[captdistraction]

I definitely was checking for knockback and wasn’t occurring. Subsequent presses didn’t change feel like normal knockback would


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[Grant 302]

Could it have been pads knock back you experienced with your stock GT booster?

How's the seal between the master and the booster?

I'd doubt knock back unless the hub(s) were failing.

That's probably the first seal to check.

 

[captdistraction]

hubs are fairly new/1 event.

Will check seals tonight. Trying to match up with Kohr to discuss their booster further

 

[JAJ]

If the hydraulics are air-free, the pedal should be hard if there's no boost and softer for a short way with boost. You're not describing either situation.
​

Some of your symptoms say that there's air in the hydraulics somewhere. There are three places it can hide: 1. did you bench-bleed the master cylinder before re-installing it? 2. did you cycle the ABS with a scan tool to clear it? and 3. did you cycle the e-brakes during the rear bleed to release trapped air in the rear calipers? It might be worth applying vacuum to the master cylinder reservoir to see if that draws any air out. It really sounds like there's air in there somewhere.

That doesn't take care of all of the symptoms, though, so I'd also look for a vacuum problem with the booster. First, check for leaks in the plumbing: fire the engine up, get out the carb cleaner and spray away at the hoses and fittings. Stumbling or any other change in the engine tone will tell you if you've got a leak. If you don't have a leak then something mechanical is wrong inside the booster or the one-way valve, or you might even have the wrong part in there somewhere.

 

[captdistraction]

will follow up on those, but yes to the three. bench bleed probably could have been more thorough

Pedal was hard with no boost (just no bite on travel), and soft some of the way with boost until hard (but again having to go deep in to get a good bite).

Last night repaired other issues on the car and after firing the motor and letting it warm, I did some more testing on the brakes, still quite a lot of vacuum noise on first press (not much on subsequent press unless I wait), but after I shut off the car and climbed out, I did hear a light hissing from near the master-to-booster connection for about 20 seconds. I've replaced lines, check valve, and gone through everything 3x.

I did dismount the master cylinder today (Which is the car's original, since my new replacement one was bad), and the gasket looked ok on initial inspection, but i'll harvest the gasket ring from the new one that had failed. Should there be another gasket at that spot?

Also, I moved one of the hard lines slightly, it was pressed against the master cylinder brace (an aftermarket product) slightly, but not enough I feel to be causing my issues.

 

[JAJ]

Hmmm... well it sounds like there's no air in the system - I'd interpreted "no bite" as "no pedal" and I had it wrong. Sorry!

So, if it's not air, then you're down to two possibilities - one is that the ABS system is crossed up and the other is a vacuum booster problem, which you're already chasing. I'm bringing up the ABS because it sits between the master cylinder and the wheel cylinders and it has a wide range of ways to interfere. How tough would it be to reinstall the original street ABS controller? It might remove one more thing that could be wrong. I see that the Boss 302R/S ABS controller module doesn't have published instructions - I wonder if there's any possibility that there's a wiring mismatch or something.

There is one more possibility and that's a mismatch between the front caliper piston sizes and the S197 master cylinder bore for the front brakes. It's unlikely though, since Ford doesn't usually stray far from the baseline. Do you know the piston sizes for the S550 calipers? I've got a database of all of the S197 OEM and aftermarket brake calipers, but nothing on the S550. If you post the piston sizes, I'll match them to the S197 sizes I have. I don't expect to find a problem but it might be worth a look.

 

[JAJ]

BTW - you asked about the gasket - I don't know because I've never had that part of the system apart. I know the caliper/rotor end of the brakes pretty well, but I've always left the OEM stuff at the head end alone. Someone on this forum must know. @Fabman?

 

[JAJ]

I did a little digging on the S550 PP brake caliper piston sizes. It looks like they're 36mm pistons throughout, which is about 10% larger piston area than the OEM Boss 302 calipers. Now, that's ok because it won't upset the brake balance very much - combined with the larger rotors, it increases the front brake torque by about 15%, which isn't a major problem. You can change it more than that just by switching pads.

However, it will lengthen the master cylinder stroke a bit too because you have to move more fluid volume to close the gap between the pads and the rotors. I have no idea how that plays with the braking. It could be irrelevant (most likely) or it could be a problem. If there's a stepped master cylinder, then causing a longer front brake stroke while leaving the rear brake stroke as-is could shift the brake balance. Theoretically, at the normal pedal travel, the rear pads would make firm contact the rear rotors while the front pads still have a ways to go. You'd have braking from the rear and nothing from the front.

While this is a lovely theory, my gut says that it's not a problem and keep looking for a booster issue.

 

[captdistraction]

Updates on my side:

I pulled the gasket from my new (but damaged MC) and replaced it on the old MC. Didn't make a change. Went through and spaced out the blowfish bracket from the strut tower, which coincidentally 1/2-5/8's spacers puts the stop right in the middle of the master on my car, and gets it away from the braided line from master to ABS.

I haven't road tested it yet, but testing the car for vacuum boost still isn't a very positive result. I took some video (and will upload here shortly), but testing the brake with the car off and foot on brake (then starting car to see it drop) and the reverse didn't yield much. it does add boost, and when turning the car off the pedal does forcibly lift, but there's definitely not enough boost. Going to put a mechanical boost gauge on the car and look at what sort of vacuum I'm pulling.

I also ordered the Kohr motorsports hydraulic kit: https://www.kohrmotorsports.com/product/brake-hydraulics-upgrade-11-14/ to just blanket replace everything and have it come from one source.

 

[Mad Hatter]

I did a little digging on the S550 PP brake caliper piston sizes. It looks like they're 36mm pistons throughout, which is about 10% larger piston area than the OEM Boss 302 calipers. Now, that's ok because it won't upset the brake balance very much - combined with the larger rotors, it increases the front brake torque by about 15%, which isn't a major problem. You can change it more than that just by switching pads.

When it comes to piston area and stroke, I believe that its a 6 pot system vs the Boss brembos 4 pot system?? But if it worked before then it must be fixable! Hope the new Kohr gear works out. Really want this brake system in the future.

 

[JAJ]

When it comes to piston area and stroke, I believe that its a 6 pot system vs the Boss brembos 4 pot system?? ...

Yes. The Boss 302 has 4 pot calipers with 2x40mm pistons and 2x44mm pistons for a total piston area of 5554 square millimeters and the 6 pot calipers have 6x36mm pistons for a total area of 6107 square millimeters, which is 9.95% bigger than the 4 pot calipers.

 

[captdistraction]

I imagine that 10% uptick in volume is where the extra travel comes into play to generate the same levels of torque. As to why however I can't get enough boost to use it is another story. Hopefully one resolved on Wednesday.

 

[JAJ]

The extra travel won't be a lot, though - if your foot has to go 2" to firm pedal with the stock kit, it's 2.2" with these, and you'll never notice it. There's a two other factors at work too that reduces pedal effort - for the same brake line pressure, the larger pistons produce 10% more force on the pads, and because the rotors are larger diameter and the braking force is applied further from the axle, you're getting 15% more brake torque for the same line pressure. All in all, I doubt that the larger pistons are a factor in the problems you're having.

 

[captdistraction]

The eagle has landed. Hopefully this and the lines are a fix. Certainly an improvement in the line design


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[Mad Hatter]

Is it just the angle or is the fluid container bigger then stock??