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Rx7 brake problems


wvumtnbkr
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Okay, you may have read the brake upgrade thread.  We did all the upgrades and still had extremely similar symptoms.

 

Here's the entire story....

 

Engine swap that is helping us drop multiple seconds off our lap times and averaging 10 to 20 mph higher straight speeds.

 

Raced wgi earlier this year.  I noticed a little higher than normal pad wear.  We had minimal brake ducting and nothing really attached to the rotor.  Air was just sort of sent the direction of the brakes.  No brake failures, no squishy pedal, nothing out of the ordinary.

 

When we got unpacked a few days after the race, the master reservoir was empty and there was a puddle of fluid under it.  It seemed to leak out of the reservoir, not the master itself.

 

I bought a new master and installed it.  I bled it by pushing fluid from each caliper into the master.

 

New pads installed for gingermen.  Destroyed the brakes in 6 hours.  All pads down to the backing plate and fluid was boiled.

 

We pit in better ducting and another set of new pads.  Same story on Sunday.  Lost brakes after 6 hours.  No pad left, fluid was boiled.

 

We changed rotors, pads (st43 front, higher temp hawk in the rear), rebuilt calipers, new hoses, added forced air ducting to the rotors in front.

 

At Pitt race, our front pads are half worn, which I'm told is ridiculous for st43.  Rear pads are gone.  Metal on metal.

 

We got a soft pedal early but still had stopping ability.  The fluid in the reservoir could burn you.  This is srf fluid.

 

What the hell is going on?  The sahlens guys are running similar pads and minimal ducting.  No issues.  An ls1 swapped rx7 is running hawk blues with some ducting.  They have no issues.  We ran hawk blues for years and never had an issue.  I can't believe we should be having pads destroyed.

 

We bed in the pads by doing some progressively faster laps until getting to about race pace then letting them cool to ambient temps.

 

Any and all advice is welcome

 

While I was getting my thought together for this post, it occurred to me that this could be a master cylinder issue.  It appears the problem started after changing it.  I have never heard of a master causing an issue like this.

 

Thanks,

 

Rob R.

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Sounds likely that you have brake drag using the other RX7s as your benchmark for wear. Have you swapped out calipers? Could be pistons sticking (front) or pins sticking (rear). The only way I could see it being a MC would be if it's not fully releasing.

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Yea, exactly.  We just rebuilt all calipers and all calipers are doing the same thing before and after the rebuild.  

 

Also, they are all wearing evenly.  I would think that can't be an individual caliper issue....

 

I'm not dismissing that idea, it just seems unlikely given the changes made and the fact that all pads are worn evenly within each caliper and versus the other side of the car.

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This is probably a long shot.   Did your method of pushing fluid back to the Master ( is that a normal way of bleeding a master? ).  Also push crud from the lines back into it?  I know pushing fluid back like that is a no no on newer ABS cars because you can get gunk in the ABS block.  

 

Is is the master cylinder seeing more engine heat with the swap ?   It's on the cool side of the rotary right?

 

Just a thought  hope hope you get it solved soon   

Edited by JDChristianson
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Totally possible.  I've never bled a master in this manner before.  It was the easiest way I could think of to do it.  The master didn't come with those little plastic fittings to bench bleed....

Edited by wvumtnbkr
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If you are getting even pad wear, it sounds like residual pressure in the system , either at the master of junction block or something.

 

Quick check would be to spread the pads in the caliper a bit, stomp on the brake a few times, release and then try to spin the hub/rotor.  If it is high effort to spin, open the bleed valve.  If it frees things up and you can spin the hub with less effort, likely residual pressure.  

 

More accurate check would be In-line brake pressure sensor, which can tell you how much pressure the system is holding.   Just a little bit, enough to keep the pads on the rotor face, will over-heat and wear the pads quick.

 

 

^^

11 minutes ago, tommytipover said:

How 'bout free play at the pedal/MC pushrod? Maybe the new cylinder needs some more?

This sounds like a very plausible reason - if the pedal/rod setup isn't allowing full release, it is essentially the same as riding around with your foot on the brake.

 

 

 

 

re: bench-bleeding, taking some old brake hard-line / ends (or buy one of those short sections of repair tube with ends), with a small plastic tube (like the one you'd have on the bottle for brake bleed) works really well for bench-bleeding.

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Before reevaluating your entire brake system (aside from the master pushrod check) Do not assume 43s or 47s will universally work on all cars.

 

When we tried those aforementioned compounds they wore like crap in the front, ruined rotors and  brought never seen before heat into the rears, after rebuilding everything wrecked by heat, the only change made was to ditch the brand and go back to the previous pad combo.

 

In hindsight, repeating as to bang this home and try and get your head wrapped around our previous conversations.

 

For us it was not enough bite and forcing the issue, the brakes almost worked well enough to stop as we had grown accustomed to but  heat went nuclear all over the car, pads that wore like iron for everyone else burned down fast and destroyed our first set of rear caliper seals in 30+ races
 

Pushrod check, jack or whole car in air, have someone stomp the brakes while spinning a tire, see if it releases quickly, get everything as hot as possible then repeat while scalding hot.

Edited by Team Infiniti
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Is any part of the brake system close to a heat source?  Have you checked every inch of the hard lines for pinching or other deformation?  What do the discs look like?  Any temperature data on the discs/pads (temp paint/strips)?

 

Dragging/residual pressure at track speeds would likely melt the calipers and crack 1-pc discs in a hurry.

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43 minutes ago, Team Infiniti said:

Before reevaluating your entire brake system (aside from the master pushrod check) Do not assume 43s or 47s will universally work on all cars.

 

When we tried those aforementioned compounds they wore like crap in the front, ruined rotors and  brought never seen before heat into the rears, after rebuilding everything wrecked by heat, the only change made was to ditch the brand and go back to the previous pad combo.

 

In hindsight, repeating as to bang this home and try and get your head wrapped around our previous conversations.

 

For us it was not enough bite and forcing the issue, the brakes almost worked well enough to stop as we had grown accustomed to but  heat went nuclear all over the car, pads that wore like iron for everyone else burned down fast and destroyed our first set of rear caliper seals in 30+ races
 

Pushrod check, jack or whole car in air, have someone stomp the brakes while spinning a tire, see if it releases quickly, get everything as hot as possible then repeat while scalding hot.

I get that it could be pad related.  However, it did this with the same compound we have been running for 5 years with no ducting as well.

 

When I unload the car, I'll try the jack it up and mash on the pedal and spin test.

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Just now, Bremsen said:

Is any part of the brake system close to a heat source?  Have you checked every inch of the hard lines for pinching or other deformation?  What do the discs look like?  Any temperature data on the discs/pads (temp paint/strips)?

 

Dragging/residual pressure at track speeds would likely melt the calipers and crack 1-pc discs in a hurry.

Rotors are now purplish.

 

No cracks.  No major brake dragging.   It doesn't feel like the brakes are dragging while in the car.

 

I have temp strips on the calipers.  They went plaid.....

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2 hours ago, wvumtnbkr said:

Engine swap that is helping us drop multiple seconds off our lap times and averaging 10 to 20 mph higher straight speeds.

 

9 minutes ago, wvumtnbkr said:

same compound we have been running

 And now a compound we have directly experienced the same symptoms with, their engineers could not explain....

 

This is where what you knew and what is currently happening have diverged, thus my stance.

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^There is lots of truth to this.  Assuming you still need to achieve the same corner speed, you are asking a LOT more of the entire brake package with the increased end-of-straight speeds.  More decel needed, more heat will be produced.   Even without a mechanical issue, you still need to deal with the heat generated.

 

 

Are those fans you are using for brake ducting pushing enough air to be effective without creating a bottle neck and actually reducing flow once you get to a certain speed?

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4 hours ago, wvumtnbkr said:

The fluid in the reservoir could burn you.  This is srf fluid.

 

28 minutes ago, Huggy said:

When I v8 swapped my s13, the headers were so close to the master it was causing heat issues there.  A heat shield and gold reflecto tape helped..

 

@wvumtnbkr What's your engine bay look like?  I don't remember well, but IIRC your master is really low in the car, and the original rotary didn't have anything hot on that side of the engine bay.  Sounds like the master could be overheating, and maybe sticking a bit once it gets hot.  

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Sooo, what i am hearing is two things....

 

1) It could still be pad choice even though other rx7s run the same pads.  And, we have tried 2 different sets of pads with the same results.

 

2) master cylinder is sticking for some reason.  Maybe heat, maybe something else.

 

The pad thing is really throwing me.  Other people run st43 with no issue.  Other people run hawk blue on this exact platform with more horsepower and or quicker laptimes and don't have this issue.  I can't just keep throwing different pads at this.  Every time we do, it is a potentially wasted race weekend.

 

I'm gonna try to figure out some tests for the master and see if I can get any results.  If not, I'll look at some other pads that were recommended.

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1 hour ago, wvumtnbkr said:

2) master cylinder is sticking for some reason.  Maybe heat, maybe something else.

 

Stupid bit of brainstorming here, so bear with me - Your master cylinder reservoir is vented, yes? Could it be that your vent is getting stuck (or clogged etc etc), no longer venting, and then your braking system builds residual pressure as it gets hot and the brake fluid expands? Hence why it seems as though there's no significant brake dragging when you check (and everything is cold)?

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