Brake Pedal Firmness Problem

[jayel579]

3) Coming late to the party, but I have a trick for the persistent "slightly soft" pedal, despite bleeding. Vacuum-burp through the reservoir. Put a vacuum pump hose through a rubber stopper in the fluid reservoir opening, apply vacuum, then RAPIDLY dump the vacuum (say by pulling the stopper, NOT the line). This sends "micro-tremors" through the fluid, and may dislodge a small bubble or two that is clinging to the side of a tube or passage through surface tension. You'll need to repeat it about a dozen times or so.

I stumbled across that after replacing a throwout bearing and trying to get it all bled down. Brake pedal feel improved at the same time!

Thank you for this Dave. I have been contemplating sending you a message for some advice as I have been struggling to figure this same problem on my car. I boiled my fluid at the end of last season and did a full change to start this season. I even pulled the reservoir off the car to get all the old fluid out but I am still fighting a soft brake pedal. I am going to give this a try the next time I am working on the car.

 

[Pentalab]

UPDATE: I'm 99% sure the cause is boiled brake fluid - but not 100%.

I've been monitoring pedal feel more closely since starting this thread, and I've run 3 track events in that time. The pedal felt fine until a full day at Road America this weekend, and then it felt like crap again after only three 15-min sessions. In the past, I only bled the front brakes since they take the majority of the abuse. However, this time, I decided to bleed all four corners. The pedal feel improved substantially. After the next session, I checked the rotor temperatures with a pyrometer, and to my surprise, the rears were just as hot as the fronts. All four corners measured ~480 degF - and that was after a cool-down lap and driving back through the paddock. I have 4" brake ducts, so the front gets lots of cooling. I'm wondering if fluid boiling in the rear calipers caused this issue in the past.

As a side note, do any of you folks use rear brake ducts ? Or is removing the rear dust shields good enough ? I have only seen it once or twice in the past, with 2" or 2.5" silicone brake hoses... routed to the inboard sides of the rear lca's. It appeared to be 'doable'...and just enough slack to allow full up /down travel of the rear suspension. Now whether it's required, I dunno. Once in a while I hear about rears being cooked, but that may well be a case of old /boiled fluid /wrong fluid etc.

 

[Sky Render]

Vorshlag found the solution to be using the larger rear rotors from a '13-14 GT-500.

Sent from my Samsung Galaxy Note 4 using Tapatalk

 

[Vorshlag-Fair]

As a side note, do any of you folks use rear brake ducts ? Or is removing the rear dust shields good enough ? I have only seen it once or twice in the past, with 2" or 2.5" silicone brake hoses... routed to the inboard sides of the rear lca's. It appeared to be 'doable'...and just enough slack to allow full up /down travel of the rear suspension. Now whether it's required, I dunno. Once in a while I hear about rears being cooked, but that may well be a case of old /boiled fluid /wrong fluid etc.

We monitored rear caliper temps and IR checked rotor temps (I know, that isn't ideal) on our S197 with the 13.8" rear rotors and did not see a need for brake cooling. The FRONT is where the heat becomes an issue... we saw (and I ignored) 500+ degree temps at the calipers, which eventually boiled the fluid, when we ran only 3" front brake cooling ducts. Normally this is enough cooling but we were ... pushing the limits a bit.

We had some folks question our lack of rear cooling, but they didn't have any better data than we did. The move to the 13.8" Rear rotor extended both rear rotor and pad life by a factor of 3. We ran no brake dust shield out back and never noticed evidence of boiled fluid in the rear lines.

Obviously there are exceptions to this rule of thumb: the AIX Mustang from Paul's Automotive was likely one of the fastest road race S197 ever built, and made our TT3 Mustang look slow in comparison. At this event at Miller Motorsport park one of my employees got a great shot of their car's front AND rear rotors glowing red in the braking zone into T1. In full daylight!

This was after a LONG straight on their "big outer" course, where my car was touching 155 and using a LOT of brake - and the images of our Mustang (see above) in this same braking zone never showed any glowing from either end. Their car must have been doing a lot more speed than us! So yea, maybe THAT car needed rear brake cooling... 800+ hp, mega aero, mega sized slicks.

RESPECT!