manual brakes for 85.5 n/a car

so ive started working on braking system again. im working on my dual master cylinder mount, hoping to have it all in by early next week.

one thing has been still bugging me though, and thats how i was always able to press the pedal, lift it and hit it again and the brakes would get a ton better. i had also noticed the pads retracting quite a ways. been doing som research on this and found an interesting post on pelican parts. in that thread a guy was having problems with his freshly rebuilt 944 s2 calipers retracting the pistons too far. the described method was the ultimate fix for the problem.

I just had a discussion with a very experienced Porsche technician. He had an interesting solution. He said he's seen this a number of times in the past, and he has seen the solution written about in Excellence, perhaps by Bruce Anderson. Here's what happens: with newly rebuilt brakes, the piston seal (not the dust boot) is apparently more aggressive in its tendency to retract the piston. The square o-ring is actually intended to retract the piston somewhat, but it retracts the piston a bit too much while it's brand new. The fix is to put some old, thinner brake pads in the calipers and depress the brake pedal a couple times until the pressure builds up and the slack is taken up. This extends the pistons farther than necessary for the "new" pads that are being installed. Then remove those thinner pads and carefully retract the pistons just enough to get the new, thicker pads to fit in. You want a tight fit -- almost so tight that a plastic mallet is needed to fit the pad between the pistons and rotor. This will eliminate the gap between the pad and piston, with the piston in its fully retracted position, at least for a while while the brake caliper seals "break in." Once they break in, there won't be the tendency for the seals to retract the pads so far.

this just made a lightbulb go off in my head and it all makes sense now. my take on it is the new square cut seals tend to stick in the caliper bore, and when the pistons move the seals just deflect, and when the brakes are released the seals pull back the pistons. when you put the thin pads in it will force the pistons out, and pushing the pistons back just enough to get the pad in allows the seals to be in their neutral position.

im not 99% sure this is what most of my braking issues were. not saying i wouldnt have had more issues with the wrong sized master master and brake bias, but i may have been able to make the 3/4" bore datsun master i had work with the adjustable prop valve. dont really care, im moving on with the dual master setup and looking forward to great working brakes.

so thought id throw an update at this. i have my dual master cylinder setup in the car now, its all bled. initially i had the same problem i had last year, except it was a little worse. i had a soft pedal, if i pumped it quick it would go rock solid and not bleed down, and releasing the pedal for a few seconds and could do it again. had a closer look at the calipers and i could rattle the pads. i tried the method above, removed the pads and got the pistons out a little, and moved them back just enough to fight the pad in place. after doing that the pedal is solid and consistent, it no longer "pumps up", its the same ever application. of course i havent driven the car yet, but im now confident the problem is solved.

so if anyone has the same issue, thats how you fix it.

Makes sense to me. Recall a similar issue when I resealed my bigreds but it went away after the first track day. Your DMC setup looks great, BTW. Building in a firewall brace was a great idea.

Rich

i have read that the issue should have fixed itself after some driving, but i put 1500km on it last year and it never got better. at one point i had things apart and used some old pad backing plates to shim the pads, and i seem to remember it helped a little, but the pads were still loose.

imo a firewall brace with manual brakes is a must on any car

finally insured the car for the summer and thought id throw in an update.

so the update, after all this work of installing the dual master cylinder setup, brakes still suck. they are better, i no longer have to double pump, the pedal is always consistent. biggest thing, they have no initial bite.

after some driving to clean the rotors up (they had rusted up over winter but i cleaned them real well) and re seating the pads a little, it started to get a little better. i feel ive got the bias dialed alright, although fine tuning is still needed. i am able to get the car to stop decently hard (hard enough where the oil light came on for a second, so thats another issue i need to sort out), the biggest thing is the lack of initial bite. once the brakes get some heat into them (2 or 3 70-10 km/h stops in a row) they start to come alive, but once they cool for a few minutes they lose the bite.

im thinking the ebc red stuff pads may be the cause. i think my master cylinder sizes are good, i was on the fence between 3/4 and 13/16 on the front and decided on the larger, but im able to dial the bias for way too much front pretty easily, so i think my master cylinder choices are alright.

any thoughts?

Yeah, pads make all the difference for bite. I run the Pagid Black track pad, high friction intended for unboosted brakes. They are murder on rotors and the rotors I use are the 928 GTS. $$ oh well.

For the street, you'll want a high friction non-sport pad. The sport pads try to bridge between street and track, giving up bite for higher temp handling. Maybe try the Hawk Blues.

Rich

the problem is the lexus ls430 calipers dont have a lot of pad options that i know of. really all i can find is the ebc green/red/yellows, and carbotech. im just having a hard time pulling the trigger with carbotech because of the cost, almost $200 for a set of pads, where every other pad i look at is less than half that.

anyone know of any other pads for the ls430 calipers?

I have used Carbotech brakes forever and really like their pads.

If you don't mind an aggressive pad that may dust and squeak a bit, then I really recommend the XP-8. They consider it their entry level race pad, but it can be driven as a crossover street track pad. Really good initial bite with a nice flat friction curve across the heat range. They the XP-8 will fade a bit from heat if you really dump heat into them, but it takes quite a bit to make that happen. Nice grippy, predictable pads that work well over a wide heat range.

I cooked the Brembo calipers on my Evo from red to purple using them, and the XP-8 compound still would stop the car, even after soaking up enough heat the pads were visibly smoking after I pulled into the paddoc. They are a bit hard on rotors, but they are great for stopping, even when cold. The time I experienced a mild bit of fade was at a track on a car where the front bads were at about 50% life, so I am sure that played a part. That would reduce the volume of pad material available for soaking up heat from each braking event.

Website: http://www.ctbrakes.com/

i did get the brakes a bit better by changing the pedal ratio. right now its in the neighborhood of 7.8:1. i have another hole (3/4" higher than stock) which would put me at 9:1. i may give it a go just to see what its like, going back to the calculator with the same pedal input force and travel 9:1 would be the same as going with a 5/8 front and 7/8 rear master cylinders. i dont want to keep the pedal ratio that high though, im worried about the stress on the firewall. i dug around at work today and found a tilton shorty 3/4" master cylinder, so i may give that a try on the front (replacing the 13/16).

now that being said, im still thinking about pads. as its driveable, the brakes dont really have much bite. any experience with the carbotech ax6 pads? they look like the heat range is a little lower than the xp8, possibly more friendly on the street. im not worried about dust or noise (as long as its not excessive squealing), i just dont want to end up with a pad that needs heat to work.

I've driven the XP-8 in everything from -30 f to +105 f. The compound has a really flat friction vs temperature curve. I did notice that very cold weather seemed a bit harder on the rotors with that compound, but I never had problems with feel or bite.

I am not familiar with the AX6 pads, but if they are anything like the old Bobcat formula, I would pass. The Bobcat pads were a "crossover" formula that was ok for the street, but I found them pretty easy to fade on a track. I pulled them at about 2/3 life on the fronts, switched to XP-8s, and was a very happy camper.

I did eventually move to even more aggressive compounds, but unless you are blowing out of the top of the heat range with the XP-8, it is a really good pad. Just dusty and a bit of squeal if you do not keep the caliper to pad contact points properly lubricated, or cook all the anti-squeal off...

they dont make the bobcat compound anymore, it was replaced with the 1521. those were both labeled as a performance street pad with no track use. the ax6 took the place of their older panther plus compound and its labeled as an autocross pad with a temp range of 150-1250*f. they say they are for autocross and many people use them for the street even though not recommended.

im not going to be doing anything on the track this year, im hoping my health improves and can next year. that being said, all ive done for track racing is some autocross (unless forza and need for speed counts). my car will always be a street car, and im not against swapping pads when i progress to that point.