eclou

Since upgrading to front Big Reds and the 5/33 bias valve in my 86 951, I have not been able to get a firm pedal - always sinks a bit before biting anything. I have bled many times with good fluid - GS610, Motul RBF600, and use SS lines. Also has a brand new master cylinder. Any tips?

Kool

Do you get a solid pedal when bleeding? I assume that you are bleeding with the ignition off.

dand86951

Kool, what would the ignition have to do with this? The 86 doesn't have ABS.

Eclou, my S4 brakes on my 86 have a firm pedal so if you can't get your big reds to firm up just take them off and send them to me and I'll send you my S4's for even swap.

Bri Bro

Sam Lin has recommended going to a 25mm master cylinder The link below has a 25mm used on a BMW. The front brake cylinder is larger then the rear so that needs to be factored in to a replacement master cylinder to keep the rears from locking up.

I would be interested in hearing from someone who has done this. I know that 333pg333 had this problem and was asking for a solution last year. My pedal is OK, a little softer then before but I do not have ABS.
http://www.e30m3project.com/e30m3per.../25mc/25mc.htm

Bri Bro

Looks like the front cylinder 23.81mm (15/16") and the rear cylinder is 20.64mm (13/16")


From the ATE site.


Master Cylinder
Trade Numbers: 010025
Braking / Drive Dynamics: for vehicles without ABS, Ø1/ Ø2: 23,81/20,64 mm, Number of ports: 3, Thread Size: M10x1, Length 1: 21,1 mm, Length 2: 159,8 mm

333pg333

Yes I've been down this path before and I don't think it's mutually exclusive to BReds either. I tried to get Sam Lin to expand on his post but he's not very forthcoming. I pursued the larger m/c with a number of sources. I eventually got a model number for it and bought one from Ebay. It's now in Evil's garage and hasn't been installed into anything. Quite likely it's the wrong model or size from what I can gather.
What brake rotors do you have Eclou? I have just changed over to the 965 drilled disks and hope to eliminate what we believe could be the cause for the soft pedal. There can be a build up of gases between pad and rotor which can affect the feel/travel of the pedal. I am due to do a track day tomorrow with the latest setup if it's not raining.
My setup is BR fronts-S4 rears, s/s lines, SRF fluid, 5/33, tube air ducting, either Pagid Blacks or PFC pads and a new OEM m/c.

eclou

I run the 322mm stock 993tt rotors with adapter hats. I think I have one of the last kits that Jason at Paragon had. I have no problem locking up the brakes, but would like a stiffer pedal/less travel. I run the 5/33 and Pagid Orange pads F&R with Broadfoot cooling ducts in the front

Paseb

hope i wont get the same problem as you when I install mine

eclou

I am now wondering if it is the 5/33 bias valve messing things up. If the bias valve is directing more fluid pressure to the rear, would it be the same as running a smaller master cylinder to the front?

333pg333

They're a drilled rotor yes? Sam Lin's point was that by going up in caliper piston size then you need to compensate with a larger m/c volume. I've yet to see anyone who has successfully done this though?

eclou

Yes, but perhaps if I dump the 5/33 bias valve there would be effectively more m/c volume directed to the Big Reds. I think I am going to try this first

dand86951

Gene, I think you are having problems prior to the brakes getting heated up on the track, but if it is a soft pedal after some track usage then my past experience with the Pagid Orange would lead me to tell you to go to Pagid Yellow or Black. I never could get the Orange pads to feel solid after even a few laps. Went to the yellows and it was much better, and the PFC 01s were better yet, but way too high initial torque for my non abs 86.

Bri Bro

I had to go to more aggressive pads on the rear due to too much braking in the front even with the 5/33 bias. I was getting a far amount of movement under heavy,high speed braking, nothing that couldn't be handled but it made things interesting. I was told you might have rear brake lockup problems with the 5/33, hasn't been a problem with me and I don't have ABS.

333pg333

I would try the PFCs also. I use 01 f and 97 r. These are awesome pads.

Techno Duck

Also experiencing the same problem on my friends n/a. Its an '89 n/a with ABS. I put turbo rears on the car and used factory new big blacks on the front with slotted GTS rotors from Frozen Rotors. Front pads are from BHP, more of a street performance pad. My friend sourced the parts, and also bought a master cylinder from a 930 Turbo i believe, i can double check for anyone interested. The cylinder size is supposedly larger and mounts up without any modification to the brake booster. However the only thing that had to be addressed was the rear brake port is mounted on the other side of the master cylinder, so i had to make a pretty basic custom hardline to adapt it to work. We also used the 5/33 bias valve.

I have bled the brakes on this car well over 4 or 5 times using a power bleeder and cannot get the pedal to stiffen up. Pedal feels fine with the system pressurized with the bleeder. Its really a pain. The point i am at now is the first application of the pedal feels spongy and sinks a little lower than what is normal, however pumping it a second time and it feels perfect.