Danielheff

Hi everyone, I have a new to me 85.5 race car and the brakes were like a light switch when we first got it. No modulation at all, light pressure doesn't do much then theres a threshold feeling that when you press a little harder the brakes bite super hard. They felt better once we put all new pads and rotors, but there's still that strange pedal feel. It feels like there's no 0-30% brake pressure. All 4 corners are pressure bled and confident there's no air in the lines.

On track it wasn't as much as an issue, but it feels like something is off. Do you have any tests or suggestions?

willrobinson

Is it on standard servo and master cylinder? Sounds like maybe a rod is going over centre or something

Dr. 914

did you replace the flex lines?

Danielheff

From what I can tell they're stock. I did a little driving around with the vacuum line clamped off and it was obviously vary hard to press the pedal, but it seemed linear.

Danielheff

I didn't replace the flex lines myself, but they've been replaced with stainless braided lines.

harveyf

I've never opened up a mid 80's vintage brake booster but they all work on the same principle. There are two chambers, separated by a rubber membrane. Both chambers are under a vacuum. When you step on the brake pedal atmospheric pressure air is introduced into the (typically) drivers side of the booster, which serves to apply 14.7 psi times the area of the booster as an assist to the force from your foot. They all also have some sort of modulator mechanism that keeps this action from being just an on/off type switch. This is a mechanism that introduces the atmospheric air in a fashion that is porportional to your pedal travel or force. If you don't find a simpler solution, I suspect that part of the mechanism has gone wonky on you. Unfortunately (I used to own a early 85 car) getting the booster out is a royal pain in the ***. But if nothing else works, you may have to source a new booster.

I also at one point tried cutting off the vacuum to my booster, to see if I could live with unassisted brakes. It was immediately obvious that wasn't going to work!

Danielheff

Thanks Harvey, that all makes sense to me too. It does seem like a booster issue, thanks for the reply!

Zirconocene

Here's an expensive (but rad) option: https://www.chasebays.com/products/c...41700242718889

Cheers

Oddjob

What pads (brand/compound) were on it and what did you replace them with?

V2Rocket

what if i told you i put together a 6:1 pedal ratio, manual brake setup with a (different) new MC that still uses the 944 reservoir for around $100 plus a few hours in the garage... lol

Zirconocene

@V2Rocket : Even more rad, I say.

This is all part of my paranoia around keeping these cars going. Critical components going NLA (or even becoming super expensive, even if the economics around it is sort of understandable) is confounding.

Cheers

willrobinson

I'd ask if you would share any details/pictures? as that is exactly like what I want to do, but in a RHD
How is the pedal travel?