I have a 1978 Track car that I installed front Big Red (GTS style) calipers with S4 rear calipers and stainless lines. I also installed an updated 10" booster and new master brake cylinder from a GTS type car. I ran the brake lines for a front /rear bias instead of the old diagonal system. Also have a Tilton proportioning valve monitring the rear brakes. The problem is the brake pedal has a long travel and a soft feel regardless of the multiple times the brakes have been bled. The car stops OK, but the pedal travel always feels squimish. Does anyone else have this issue with the GTS brake conversion?

The S4 style master cylinder has a stepped bore. Would an older style master with a larger bore size help the pedal feel? Any suggestions?

 

Is it an auto or a manual?

Which brake fluid do you use?

What technique did you use for bleeding the brakes? Sometimes those devices that suck the fluid out of the bleed nipples are better to get out the last drop of air.

Marton

 

Did you run hard lines? <yes I know this may be a stupid question>

 

The car is a manual transmission. The new hard lines are copper alloy similar to the OEM. Then have stainless braided line to the caliper. The brake fluid is the good stuff (forget the name). About $15 a can. Used a Power Bleeder....and also used the old fashion two people method. I talked to the vendor of the Big Brake Kit and he has no solution. However, when you talk to ANY vendor about brake problems, they really don't want to get involved.

Thought about a different master with a larger bore. This sometimes gets complicated. Or maybe a (residual??) valve to keep more fluid in the line.

 

Soft pedal just sounds too much like something is compliant, hose expanding, air bubble. When you press down fairly hard, let up just a bit, and press down again is it still feeling soft?

 

Not sure I have good news. My 79 has S4 brakes - they are soft compared to driving an S4. First time I drove a stock S4, I almost kissed the windshield the brakes were so instantly on vs. my 79.

IIRC the brake bias is much different early to late cars. The early cars have a much more even distribution of braking power. I learned this when Z informed me he installed an early brake bias into his S4 to even out the braking. I’m not sure you would want the S4 bias if it would make the brakes that touchy since the early cars do not have ABS.

 

It could be possible that there is too much clearance between the booster shaft and the piston of the master cylinder. Are all components for the same vehicle ?

 

I know that this is going to sound really silly, but I've actually fixed half a dozen cars over the years with this problem after new brakes were installed by others.........are the bleeders on the top?

You think I'm kidding?

greg brown

 

.... and if so, was the inner bleeder vented first? Not to be insulting - just going over potential omissions:

My spare set of 993TT big reds require the crossover pipes and bleeders to be switched if fitted to a 928, so Greg's point is a good one.

 

Yes, The bleeders are on top. Yes I started at the inside. I was thinking that the ABS on S4 cars may have an effect on pedal pressure .

Any other Big Brake Converts have an opinion?

I may try the Wilwood or other brake companies to get an opinion.

 

Let me repeat two good ideas above:
Remove the relay for the ABS
Check the play between pedal and master

And add another:
Your new master is weak.

And a mediocre idea:
Drive it a while and then bleed it again.

 

Lloyd,

My 79 is the same way. I did the S4 brake upgrade and they were VERY soft to begin with. As I used them, they got better but never like the old crappy 79 brakes were. With the stock setup, it alway felt like stepping on a wall as soon as I put my foot on the brake. With the new setup, there is a little more "mush" and then it firms up a little into the travel of the pedal. The car does stop better than the stock setup for sure, but you really have to push on the brakes to bring it down at speed. I thought that the later model booster would help, but after reading your post, I'm doubting that. I've also developed a slight vibration in the pedal when I get on them hard while hauling arss and think that the rotors may be slightly warped already. I've only got like 2k on the brakes at this point so I'm scratching my head.

Hacker, funny you mention kissing the windshield. I drove Marc White's 88 at the 3rd coast a few weeks ago and I layed my foot on the pedal at the first stop I came to and almost did the same thing!!
The later cars (S4 and newer) stop on a friggin dime! A lot different than the 79 even with the upgraded brakes.

Let us know how things go. Good luck!!

 

No thoughts on the brake bias changes? If the S4 bias is sending 75% of the power to the front vs. 60% like the early cars do (making up numbers, cannot remember the exact figures) this would change how the car feels.

 

I always do outside first for bleeding on my 911, and when I redid my 928 brakes. The outer segment is farther from the M/C, and is what I've been taught to do first.

 

10" from 86.5 - up?

I was reading the service bulletin for '86, and it mentioned a new ratio in the booster for the 86.5 (S4) brake system. Also a new brake pedal, without a stop, as the new booster has an internal stop.