Pedal softening during race

#1

Dad’s car has a new master cylinder and 4 rebuilt calipers. Bled probably 5 times. Pedal starts out fine and softens bad during the race and rears start to lock up and pull one way or another. ABS is functioning as far as I know but he says the rears are locking. Would a sticking caliper cause the peda to go way soft? Other ideas?

#2

I had a similar issue and it wound up being a defective ABS pump. When I first bought the car, it would just be squirmy under braking and I could drive around it. Then one day, barreling down the front straight at CMP and braking for turn 1, the right front completely malfunctioned. Wasn’t getting any fluid to it at all. Was… interesting… for a bit. Anyway, replaced the ABS pump at the track and it’s been fine ever since. This is after replacing calipers all around, multiple bleeds, etc. trying to figure out the source of the problem.

#3

A sticking caliper could cause a brake to overheat, which would give a soft pedal and cause pull. Shoot an ir thermometer at the rotors when he comes off track and see if there’s a big difference at one corner. Check the pads for weird wear patterns.

#4

Just because a person thinks that a rear caliper is locking up, doesn’t make it so. Many a time I’ve heard folks convinced that their rear brakes were locking up, but then they came in with flatspotted fronts, not rears.

My point is that before one can conclude that a certain corner is locking up, ya gotta scheme up a way to confirm it. Otherwise you can, in typical Scott fashion, spend days and days on what turns out to be a big wild goose chase.

#5

When the pedal goes soft, if you pump it a couple times does the behavior change? Like maybe as it gets soft, a heavy foot pressure puts the pedal pretty close to the floor. But after a couple pumps, the pedal is still soft, but heavy foot pressure doesn’t press the pedal as far down.

#6

No it stays soft. Does not appear to be a vacuum issue (yet).

#7

I’m not asking about whether or not it stays soft. I’m asking about a change in how far down the pedal goes before/after a couple pumps.

#8

Re. posts on fb re. bleeding ABS pump and bench bleeding the MC. Just to prove that consensus doesn’t exist in racing, let me be the counterpoint. I don’t agree that bleeding the ABS pump nor bench bleeding the MC is necessary.

I went thru an adventure a couple years ago where I was convinced that my braking power had dropped significantly. As part of that effort, I started measuring hydraulic pressure at each corner. Measuring hydraulic pressure turned out to be harder than I thought, and I went thru some hoses that behaved as if they were partially plugged so the effort was a long hard slog of chasing after one wild goose after another. I bought some spare ABS pumps and changed all 3 several times trying to understand why the results were unpredictable. I measured hydraulic pressure at every point in the brake system that I could disconnect over and over again trying to understand why my readings were occasionally goofy (turned out to be a bad hydraulic hose).

This went on for weeks as I struggled to get the gauges to consistently make sense. In all of that effort, I must have bled the damned system dozens of times, and played musical ABS pumps, bias valves, and MCs. I was up to my elbow in brake fluid night after night.

But never once did I really have a problem bleeding the system. That part always behaved predictably. There was never a need to do anything special to bleed the ABS pump, nor bleed the MC. The usual tried and true way of bleeding always worked.

That said, I’m now quite wary of trying to provide info to anyone because SOM just beat the crap out of me. I’m not saying “I’m right”. I’m perfectly happy being one data point among many.

#9

If we are going to use everyone as a data point why did SOM’s posts disappear?

You had a spirited and helpful exchange, your egos just got in the way.

This type of exchange will surely stick a fork in this forum… or maybe it will rekindle it.

Ranger there are a lot of us that enjoy your loooong technical explanations on how to clean the windshield before a race so please don’t crawl into your shell.

SOM, pump the breaks!

#10

[quote=“poppy” post=83292]If we are going to use everyone as a data point why did SOM’s posts disappear?

You had a spirited and helpful exchange, your egos just got in the way.

This type of exchange will surely stick a fork in this forum… or maybe it will rekindle it.

Ranger there are a lot of us that enjoy your loooong technical explanations on how to clean the windshield before a race so please don’t crawl into your shell.

SOM, pump the breaks![/quote]
I can’t tell if you’re kidding or not re. SOM’s posts disappearing. I’ve not deleted a post in years. Am certainly not going to delete a post of someone yelling at me. Is good for the community, I figure, for me to get yelled at in public.

I am bumming about pissing SOM off tho. I’m going end up moping about that for days. I always take it hard when I make an ass of myself.

#11

Ranger my mistake. I was looking at this thread and thought it was the “Ignition Switch and Aux Panel” one that you and SOM got into it and didn’t see the exchange anymore, so I retract the question on why the posts disappeared.

[quote=“Ranger” post=83293][quote=“poppy” post=83292]If we are going to use everyone as a data point why did SOM’s posts disappear?

You had a spirited and helpful exchange, your egos just got in the way.

This type of exchange will surely stick a fork in this forum… or maybe it will rekindle it.

Ranger there are a lot of us that enjoy your loooong technical explanations on how to clean the windshield before a race so please don’t crawl into your shell.

SOM, pump the breaks![/quote]
I can’t tell if you’re kidding or not re. SOM’s posts disappearing. I’ve not deleted a post in years. Am certainly not going to delete a post of someone yelling at me. Is good for the community, I figure, for me to get yelled at in public.

I am bumming about pissing SOM off tho. I’m going end up moping about that for days. I always take it hard when I make an ass of myself.[/quote]

#12

I’m not asking about whether or not it stays soft. I’m asking about a change in how far down the pedal goes before/after a couple pumps.[/quote]

Can’t answer that. Not sure.

#13

We may be on to something (what exactly I do not know).
2 door car we had…good pedal feel.
The two 4 door cars require more effort. Components have been swapped many, many times.

Is Oscar’s a 4 door?

If so, swap all the components just to find out it is still spongy.

Answer: don’t use the brakes.

RP