333pg333

Ok I assumed incorrectly that I have been using a DOT 5 when in fact the SRF is a DOT 4 and adjudged one of the best in the marketplace. So I will assume that the SRF is not to blame for my soft pedal.
I'll do a bit more research into the ATE and Girling brands.

anders44

are you running rubber hoses? or stainless?

I run alot of different stuff as I love to import & sell here so I have tested loads of brands, SRF definetly a favorite but it's kinda nasty for the system, just drain the calipers and change it quite often its fine, with the motul on a good 2nd, for dailydriver cars just use superblue since its cheap and better than stock stuff.

pete95zhn

Motul RBF600 here, braided SS lines. ABS, stock '87 master cylinder and pedal travel is short and firm. No pumping required and the travel doesn't change much even when the brakes get hot. BUT this requires thorough bleeding!

Then some about master cylinders and caliper piston sizes. '87 uses the exactly same master cylinder than the '88--> with bigger pistons. And '88--> ( turboS or whatever you want to call them ) piston area is the same than in Big Reds / Blacks AND even with 6-piston 996 GT2 calipers. Period.
There should be any prake pedal travel issues related to mechanically sound calipers. Just bleeding, bad fluid or spongy rubber brake lines.

anders44

flexing calipers will also do it, not that any of the stock ones are that bad at it, but you gotto love monoblocks

333pg333

Going from the S4 brakes to the BReds increases the front pad area by 36% so I guess this requires more force? I might be wrong there...
However I'm one of the ones that subscribe to the theory that as a GT3 has less power/tq than some of our cars so why should it have all the fancy software AND bigger Monoblock brakes. At least we should even it up a little bit no?

I have s/s lines too as stated before. As I have also found others with the same symptoms as me here and on other forums I will continue to investigate. Again, any help with the larger M/C is appreciated.

jerome951

If you increased the fluid capacity of the front calipers by switching to Big Reds your MC may be undersized. No direct experience, though, to confirm you need a larger MC.
My factory turboS brakes are rock hard with little pedal travel. I used ATE fluid and PFC97s.

Is the sponginess all the time or only the first application after cornering? For a while I had initial sponginess after hard right hand turns. Turned out the front left wheel bearing was slightly loose allowing the rotor to wobble and knock the pads back a little. Check this and also the trueness of your rotors.

pete95zhn

Pedal force and pedal travel does not correlate.

If master cylinder and caliper piston areas stay equal, the force delivered to pads stay equal.

Bigger pads equals more grip at same force, or same grip at less force = less heat. Bigger discs absorbs same heat to bigger area than smaller discs, so they run cooler. Also bigger disc diameter requires less pedal power to be applied for same stopping power , or more braking at same pedal power ( torque = force x distance ) and again less heat.

Rear engined Porsches use different bias valve, which deliveres more ( and earlier ) power to the rear, because rear wheels can stand more braking without locking due of more weight on wheels. I used that too with std 951 rear and Big Blacks front, but changing to 6-pistons front with 350mm discs and 4-pistons rear with 330mm discs the car become too tail happy at braking. With std bias valve it's normal again. Althougf 295 rear tires might have something to do with that too.

pete95zhn

>>'87 uses the exactly same master cylinder than the '88--> with bigger pistons. And '88--> ( turboS or whatever you want to call them ) piston area is the same than in Big Reds / Blacks AND even with 6-piston 996 GT2 calipers. Period.<<

= No more fluid capacity. Been there, done that.

333pg333

Yes I see what you mean. I was just thinking that with more pad area it would require more force but in fact it's less force. Not enough sleep.
Does anyone know how large in mm our m/c's is? eg the 993tt is 25.4mm hydraulically operated & 993 n/a is 23.8mm vacuum operated. Are ours Girling or ATE?

ENGINEERMAN

I have an 86 951 with stock MC, sst lines and Big Red Kit from Lindsey Racing with Hawk HP PLUS pads which are very grippy. I use ATE Super Blue. I have a very hard pedal that does not get spongy or fade even at the track BUT I find that I have to apply VERY heavy pedal pressure to get them to stop the car aggressively. This can get a little scary sometimes. It feels like there is not enough boost assist. I have tried the system plumbed with the vacuum increaser in the line to increase the vacuum to the MC and with it deleted with very little if any noticeable difference. I have also bled the brakes repeatedly so am sure that I have all the air and moisture out.

My son has an 87 951 with stock brakes and his pedal pressure is much lower to get equivalent stopping power.

This relates to the earlier question as to whether the 86 master cylinder is properly matched to Big Reds. Any comments as to how I can get the great stopping power that Big Reds advertise without using scary amounts of pedal pressure?

Thanks in advance!

333pg333

Ok so there seems to definitely be enough examples of people with soft brake pedals to know that I have not been imagining it. My brakes have had the power bleeding done regularly enough too. Next thing to do is test the booster under pressure for leaks.
The symptoms that Engineerman describe contradict the theory in that the smaller 86 m/c is providing him with possibly too hard a pedal. This is based on the assumption that it is indeed a smaller m/c? Does anyone know this for sure? Also the 86 doesn't have ABS so that could be something to note here?
I tried to find out more info re ATE and Girling products but didn't get far. Does anyone know if they go under some other web address?

sawood12

I think the brakes are set up with less power assistance than modern cars to give you better feel, however the feel of the pedal should still be hard and not spongy. When I upgraded to Big Blacks I did notice a slight increase in pedal travel (all other components remained unchanged i.e. brake lines and m/c) and after I confirmed the m/c is the same on the early turbo's and later turbos then I just came to the conclusion that it is what it is. I've now got used to it and find modern cars with their over-assisted on/off brake pedal action tricky to drive and give you little or no feedback as to what is happening between the rubber and the road.

It might be worth looking at the 928GTS m/c rather than the 993TT's as that car has the same brake setup as our BB/BR upgraded 951's.

333pg333

Don't think it's the same m/c setup and booster though Scott. I'll double check on that just to be sure.

ENGINEERMAN

Just to clarify, what I was saying regarding situation is that

a) There is NO spongy feeling like 333sp333 describes, nor is there excessive travel.

b) My issue is that if I am trying to stop very quickly (especially from speeds in excess of 40 mph) I almost have to use 2 feet to "lock em up" (I don't have ABS). There is no fade, I just have to push VERY/uncomfortably hard to stop quickly. IS THERE ANYTHING THAT I CAN DO TO REMEDY THIS SINCE IT IS A POTENTIALLY DANGEROUS SITUATION IN A PANIC STOP SITUATION? Please advise.

I don't have any ideas about where the spongy feel is cominng from for 333sp333. Good Luck with that. BTW, I have always had good luck with the ATE SUPER BLUE.

Thanks in advance for any advise you can give!

Tom

anders44

sounds like the brake booster isn't working all that great.