Master cylinder for boosterless setup

[danval]

So I got rid of my booster.
I'm using stock calipers front and back. What master would you recommend? 1" (25.4mm) or 3/4"(19.05mm)
Also, should I run a proportioning valve for the rear brakes?
Thanks in advance.

[nando]

I'm no expert but I think you want a smaller MC. You can also alter the pedal ratio for a similar effect. The larger MC will result in a really stiff pedal and less clamping force (the opposite of what you expect). I think the stock MC is 21mm?

There's a number of threads on this already and we have some brake experts here. I don't know if Massive Lee still gives free advice but he knows quite a bit about E30 brake systems.

[E30Whore]

I was told to move to a 25mm. I'm still looking for verification.

[HollywoodDale187]

7/8ths master cylinder. [emoji1362]

[whodwho]

I was told to move to a 25mm. I'm still looking for verification.

7/8ths master cylinder. [emoji1362]

These would make for rock hard useless brakes, without assist you will need much smaller (like 5/8 for stock calipers ) and will probably need to change the pedal ratio especially if this is a street car.

[superleggeram3]

In order to give you a better answer without guessing - need to know a couple of things:
- what pedal box/system are you using (aftermarket or stock ?). If aftermarket what is the pedal ratio
- is the MC going to run both front and rears or will you run a dual MC - one for front and one for rears ?

[Madhatter]

Fluid works opposite than people might think as its all about pressure not volume. Smaller piston results in higher line pressures, generates more clamping effort.

Going bigger will actually mean lower pressure, with decreased pedal travel, but will actually require more pedal effort to try and get them to pull up. Think about it logically, the bigger surface spreads out the load, meaning less force is actually transferred for the same pedal effort.

If starting from scratch with brake masters, good rule of thumb for all cars is to start off with a 5/8 master and go from there.

So, regardless of what your setup is, go smaller than what you already have, never larger.

Edit: yeah, you'll probably find you need a bias valve for the rears. I've found e30's run substantially more rear brake that a lot of cars. You can remove the factory rear proportioning valve if you want. I found it meant too much rear brake though. Also keep in mind if you change the valve, you'll probably need to tee the front calipers together on one port of the master and use the other for the rears.

[squidrope]

Could the stock MC be used? Pedal travel would remain the same. Pedal pressure would need to increase.
Yes/no?

ETA-My plan was to do this on my lemons car build. Building a movable pedal box and was hoping to use stock MC to keep costs down. Car should weigh 1900-2100 dry, no ABS, w/brake prop valve.

[Northern]

^ Look at how the Massive delete kits alter the pedal ratio to retain the stock master. That's the route I would go.

[bimmerteck]

So there is more to converting to manual brakes than simply removing the booster. On the M3 We removed the booster, changed the pedal ratio, swapped to a smaller diameter master cylinder, changed the brake pad compound, and installed an adjustable proportioning valve.

On my e30 'vert the same as above sans the adjustable proportioning valve and I was already running the proper master for my new pedal ratio.

[squidrope]

^ Look at how the Massive delete kits alter the pedal ratio to retain the stock master. That's the route I would go.

Looks like he changes the pedal ratio from 4:1 to 6.1:1 So there will be a bit more pedal stroke to achieve the desired braking. That's an easy fix.

Thanks

[whodwho]

You will want a smaller MC as well for a usable MC to caliper ratio

For reference I am running E46 M3 calipers with dual 16/19mm MCs and 6:1 pedal and has a normal pedal travel/feel, with stock calipers it was sketchy