2nd Gen Brakes - Soft Pedal

benspeed

benspeed

New member
I have a well sorted 2nd Gen RX7 that has sporadic brakes. I spent the entire race weekend pumping the pedal with my left foot just to keep some firmness - if I didn't it went nearly to the floor. Kinda disconcerting.

Bled the brakes multiple times, new bedded pads. Is this a sign the master is going?

Thanks,

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BenSpeed
#33 ITS RX7
BigSpeed Racing
[email protected]
NNJR
 
Actually, more information might clarify the problem:

...Did you have a SOLID pedal at the start of the race, and have it degrade during?
...Is it a new problem?
...Does the pedal get hard AFTER being pumped?
...Does the pedal sort of move toward the floor after pressure is applied...hard, but moving?

Nothing will help locate the problem unless the system is intact...brakes must be adjusted, and the system bled properly BEFORE the event. I mean here, that it is difficult chasing a problem that is essentially one of maintainance. That possiblilty must be eliminated by starting the diagnosis at the begining, with a known system.

Please write more. Perhaps someone can help.

Good racing.

Bill
 
Bill is on the right track. More information will narrow the problem down. I am pretty good with brakes. My system is tight and solid. I bleed after every weekend with Motul600. Fill me in, and I'll help all I can.

PMD
 
Thanks for the replies. Here's some of the detail:

1. The problem has existed for two races now and bleeding the system hasn't helped.

2. The pedal is hard when depressed firmly when the car is parked and running. It does not appear to gradually go down even though firm - but I will test this

3. The pedal suddenly will be soft and go to the bottom of the travel under hard braking unless the pedal has been pumped several times firmly.

4. When the pedal has been pumped a few times the brakes work fine

5. Maintenance has been very methodical

I spoke to ISC Racing and they recommended testing the master cylinder by very gently pressing on the pedal and seeing if it will go all the way to the floor - that would mean the master has an internal problem. Pumping hard would seal the the internal leak and brakes would work - slow would not seal it.

Let me know if any of this highlights another potential issue.

Thanks,

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BenSpeed
#33 ITS RX7
BigSpeed Racing
[email protected]
NNJR
 
Master cylinder is a good bet at this point. If you have no fluid leaks, then there isn't much else that could be causing it. Check all of your brake line disconnects for seepage, just in case.

I would also recommend buying a NEW master from Mazda Comp. I have had dubious results from remans you can get from big chain autopart stores.
 
If the brakes work "normally" when essentially cold (early applications, stationary with engine running), and apparently degrade when under severe use, I'd think of heat boiling the fluid in the wheel cylinders (calipers). Pumping the pedal when hot overcomes the presence of "gas" (at this time we are talking of gaseous brake fluid, not "air" as in unbled air from assembly) and super compresses it so you have a solid pedal during THAT application. As soon as you release the pedal, the gas resumes its original volume (compresses, then expands), and the pedal is again soft.

As that volume of gas cools, it returns to the liquid state, and the pedal again feels normal (solid).

Incidentally, all discussion here should deal with only regular brake fluid, not silicone brake fluid, which is another animal entirely.

You should also look for evidence of excessive heat on the brake pad material, and the calipers. I had problems with cooked seals, cracked rotors, and warped pad backing plates. Ours was very evident, but we never had boiled fluid. We also had huge ducts to the calipers.

I'd look elsewhere for the problem than the master cylinder. The most common failure of the master is that the seals leak BETWEEN the cylinders, causing the pedal to slowely move to the floor, WITHOUT any external leakage of fluid. If your cylinder is dry on the outside, and under the dirt seal, AND at the firewall where the brake booster enters the cockpit, then the problem is not the internal leakage of the master cylinder.

Solution to the boiling fluid problem is:
-Higher DOT grade of fluid which will raise its boiling point.
-Cooling ducting to the calipers, hopefully to the center of the disc for ventilated style of discs, and the edge of the disc for uniform cooling of the solid style.
-Brake fluid re-circulator system returning to the master cylinder...illegial in IT, I believe.
-Water spray bars on the disc. I saw them used on an ITE car for an enduro...big tank in the back seat.

That's about all I can think of that this time. Hope it helps.

Good racing.

Bill
 
Thanks Bill - the pedal goes soft when the car is cool - warmup laps. Haven't seen any sign of excessive heat issues. I've got big ducts to cool the rotors also and use Motul race fluid. No signs of cracking or other issues.

I've got a friend coming over to look at it tomorrow night - I'll let you know what he finds.

Cheers,

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BenSpeed
#33 ITS RX7
BigSpeed Racing
[email protected]
NNJR
 
BenSpeed, I have a 1st gen with the OEM type master cylinder. There is a seperate piston for the rear & front connected to one operating rod. One main line heading to the rear with two seperate lines to the front. In past years I went through the normal BS of bleeding brakes untill I was blue in the face. RR, LR, RF, then LF. After the rear was bleed the pedal was hard half way down (air out of the rears) & ya could bleed the crap out of the front but never have enough pedal stroke (as soon as the rear pads hit the rotor that's the maximum stroke you will get but it is not full stroke to bleed the front.) to get all the air out (for racing purposes) of the front. Including cross bleeding & anything else I could think of. Ya might try this if the 2nd gen master cylinder is similar. This year I added speed bleeders to each caliper (not required if ya got help) & a speed bleeder at the master cylinder in the line that travels to the rear calipers. There is a soft tube attached the the master cylinder bleeder which is inserted into the master cylinder resivor (cover off)while bleeding the front calipers. Open the master cylinder speed bleeder, pump the pedal (you now get full pedal stroke while bleeding the front) bleed the RF, then LF & the air is GONE.

Have Fun
wink.gif

David

ps: Im am I guess you could say I am way to economical (cheepskate)to procure a vacuum/power bleeder.
smile.gif




[This message has been edited by ddewhurst (edited July 21, 2004).]
 
you have a bad master cylinder. been there done that. get one from mazda comp.

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steve saney
it-7 /it-a #34
 
There's nothing wrong with your brakes. Check the front wheel bearings. If there is exsesive play the wobble will push the pads back and give you the impression of overheated brakes. This is why they seem to be fine sitting still but soft after you get going. My EP second gen does this all the time and we check/adjust wheel bearings after every session. Very common with these cars.
 
Dang, eprodrx7 beat me to it while I had to go find my password. He's correct, I had the same thing happen, bad left front wheel bearing, too much slop, pushed the piston back, no brakes. Everything is fine sitting still.

-chuck-
 
I'll check the bearings again, but that's on my regular checklist and I havne't noticed any wobble. Good thing to know to check for!

Another question - when we were inspecting the brake system, I had removed the front calipers to take out my shocks (need rebuilds). My buddy pushed the brake pedal down several times and the piston on the FL caliper went out too far since it wasn't on the rotor and leaked. I pressed it back and refilled the fluid the leaked out. Likely it will need bleeding, but does extending the piston too far damage the caliper? About 3 oz of fluid leaked out from the piston when he pushed the pedal down.

Thanks for the excellent pointers. I've ordered a new master cylinder and will let folks know how the fix is accomplished.

Cheers,

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BenSpeed
#33 ITS RX7
BigSpeed Racing
[email protected]
NNJR
 
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