soft brakes 1st gen

[Rex B]

OK, I'm about to damage the seat upholstery over this one:

My brake pedal has been spongey or soft on my race car pretty much since day 1. I insist on a rock-hard pedal on everything I drive, but I can't get there with this one, not even cold and freshly bled..

I've bled the brakes until I'm blue in the face - gravity bled, and pump/release bled. It's been on the track probably 20 times, with a quick bleeding before most sessions.

Fluid is fresh and of high quality.

Front calipers have been rebuilt.
Pads are Hawk blacks

I have removed the handbrake. I thought that this loss of the self-adjustment on the rear was the culprit, but the pads are tight against the rotor and the piston, no slack at all.

I sure would like the reassurance of a solid pedal before I go hurtling into TWS Turn 1 Saturday after next.
Any ideas?
--
Rex Burkheimer
Parts Plus Marketing Director WM Automotive Whse., Fort Worth TX

[Silkworm]

Leaking or poor sealing master cylinder? Sorry, don't have a 1st gen, and haven't raced my 2nd gen yet..

PaulC

[Rex B]

Thanks, should have mentioned the master cyinder has been replaced, with a good-quality reman unit. The car stops well, but there is too much pedal travel until it starts to firm up. I'd like about 1/2 inch of travel, then a "rock". that probably won't happen, but I'd like to get as close as possible.
Unless I get a better idea I'll probably just order a new MC from Mazda, just to be sure.

[wlfpkrcn]

Do you have a proportioning valve in the drivers compartment? If so it might have an air bubble trapped in it. Do you run steel braided or rubber lines? The stock Mazda rubber ones have pretty good reenforcement. Aftermarket rubber lines may flex more. I use steel lines from Racing Beat they work great. However I have seen a steel lines seep at the crimp. Hope this helps keep us posted.

Eric

[TURN9]

The master cylinder has a rod behind it that will give you a little adjustabiblty. resolve to this last. Have you always had a soft pedal, or did it happen after you changed brake pads. All of the metal clips that hold the brake pads in place for a street vehicle should be removed. Make sure that the pads aren't ,pardon my french "cocked". I've seen this happen, and it will destroy the brake pad by bending the backing plate of the pad, just by bleeding the system. You are starting from the farthest away from the master, and working forward, aren't you? (R/R, L/R, R/F, L/F) Bleed the system in a bottle leaving the end of the hose submerged in the fluid.

All of this might seem to be redundant, but I would bet that you have a cocked (sorry) pad. Check 'em, and bleed the system a few times. Did the rear calipers get rebuilt at the same time? Does the fluid you are using have a high enough boiling temp?

Last result, check that Master cylinder rod. That should only adjust the pedal height. Forget I said that for now.

------------------
Corey L. Clough
ITS RX7 Soon-To-Be
[email protected]

[Rex B]

No proportioning valve (other than the OE unit on the firewall). Not allowed in our rules.

Flexible lines are steel braided from Mazda Comp, fairly new. I have had the seeping issue on other cars, this one doesn[t seem to have it. I'm not losing fluid.

Using Comp Friction 550-degree fluid.
Bleeding RR-LR-RF-LF.

One thing that does concern me is the metal tubing loops at the proportioning valve on the firewall. I think a normal bleed should purge any bubbles in the top part of those loops.
Has onyone used a commercial pressure bleeder on one of these?

[Rex B]

I do have those clips still on the caliper. I will be happy to discard them. Surely those alone can't soften the pedal feel?

And I will look for a cocked or bent pad.
My quick inspection earlier did not reveal any pads that did not appear to be flat on the rotor, but I wasn't specifically looking for it. I'm surprised that that is even possible.

[Hahn63]

Master cylinder bolts tight? Fire wall flexing? Bent brake pedal or loose?

[Rex B]

I don't think there is any mechanical slack in the linkage train. There is probably some looseness in the pedal pivot - it's a 160K miles car - but it doesneel loose, just soft. It feels like there is air in the lines.

I think my next step (no pun intended) is to have someone pump the brakes while I watch what happens on the business end.
Then I'll change the MC. In fact, I'll order that sucker right now. I should have known not to use a reman MC on a race car, but I had a long shot at getting some help from the manufacturer.

[Quickshoe]

Originally posted by Rex B:
My brake pedal has been spongey or soft on my race car pretty much since day 1. I insist on a rock-hard pedal on everything I drive, but I can't get there with this one, not even cold and freshly bled..

How strong is your leg, how hard is rock-hard? What you are feeling might be normal for a first gen. I know I could press the pedal with my twig legs hard enough to see the firewall flex. Nowhere near this amount of pressure required on the track though....lockup would occur at far less pressure.

Are you running Hawks all the way around? Still running rear drums?

[TURN9]

Technically speaking, all master cylinders are reman/rebuilt exchange. The only "new" ones come on the car when new. There is no rocket science to rebuilding one either. If you have rebuilt your calipers and bled the system, continue to bleed the system, saving the new fluid that comes thru. If it is dirty, throw it out. Sometimes I would come across a car that required multiple bleedings, then "POOF" firm pedal. No explanations

Good Luck.

------------------
Corey L. Clough
ITS RX7 Soon-To-Be
[email protected]

[Rex B]

Ok, checked all of the above, and all checked out - no mechancial issies downstream of the MC. However, I noticed the pedal was pretty good with engine off, but dropped and got soft when the engine was running. That tells me the booster is the issue. Now, is it normal? Do these fail, and how do they feel in failure mode?

[Rex B]

So, this adjustable pushrod - is this the rod from the pedal to the booster, or between booster and master cylinder, like on a Honda?
I hate working under the dash, especially with the cage.
Is there a factory stop for pedal height? Adjustable.

I know all this is in the shop manual, but it's at home and I'm at my desk thinking about things other than work

[rx7chris]

Not sure how to do it but you might want to check the brake booster.

Chris
IT7#88

[Rex B]

The guy at Mazdacomp said the boosters rarely fail, and in fact they don't even stock them in the US. Japan-order only, $185.
So I think the booster is probably working, so maybe I just need to reduce the amount of play between the pedal and the master cylinder.
I guess I will take it all apart tonight

[Mike Cox]

As much as this is a serious thread, I must comment. If you can't gid rid of a spongy pedal, remove all the ballast from your car and carry a boat anchor. Then you can literaly "throw out the anchor". Question, are you running the stock hoses(rubber)? You may want to switch to steel braded hoses and see if that cures the problem. Or as Marc Dana would say: "brakes, we don't need no stinking brakes"

[TURN9]

Great source for this problem is to contact Dave Lemon @ Mazdatrix. E-Mail Dave @ [email protected] Tell him Corey sent you. They carry many stock replacement parts for 1st Gen's and 2nd Gen's. Don't but that booster yet. Have you checked the junkyards? 1st Gens still flourish in the western regions like pancakes at a truck stop. It's hard to diagnose the problem without actually seeing it. Maybe if you take it to one of the races in your area, then a racer in your class sould look at it. Who knows, maybe what you deem to be soft is normal. I would just bleed, bleed, and bleed. Just a thought. sorry I couldn't help much more.

------------------
Corey L. Clough
ITS RX7 Soon-To-Be
[email protected]

[buzzinhornet]

Forgive my ignorance, but how do the rear calipers adjust back "out"? I saw in the first post something about the handbrake? I do not have the handbrake hooked up anymore. I just put on stainless lines, flushed and bled brakes with quality fluid, in the proper sequence, and my brakes went from reasonably firm to completely spongy. I'm thinking it's from too much slack in the left rear caliper. We cranked the piston all the way back in before putting the caliper back on. I figured it would adjust itself back out as the brakes were pumped. Is there some other method? Thanks.

[GEO46]

Are the levers where the cables attached to the calipers still there. Use a screwdriver or a box wrench, (I use a 12mm) to ratchet the levers. This will move the piston back out to the pad. It may also be necassary to start the car and press the pedal a couple of times to fully seat them. Just finished doing it yesterday on mine.

[Rex B]

Good suggestion from Geo46. This was the issue I first thought to be the culprit. The rear calipers self-adjust from the handbrake application. Remove the handbrake, and you no longer have an automatic mechanism taking up the slack as the pads wear.
Had I to do it over, I would have left the handbrake in place. It did not weigh that much.