Swapping drums to disks, master cylinder?

[E30SPDFRK]

I've read the threads on here about swapping the rear brakes on early cars (there aren't many), and people say just to keep the stock master cylinder. That seemed odd to me since you're completely changing the braking system, so I checked on realoem.com and the part numbers are different between drum and disk cars. It lists the early drum cars as having a 19/20mm master cylinder and the disk cars as having a 22/17.5mm.

Does anyone have any actual experience running the early master with disks, or is this one of those "I read it online somewhere so I'm going to tell everyone that it's the truth" things? Another thing that seemed odd to me was the difference in master size for the front brakes between the two, considering the front calipers have the same size pistons.

[jlevie]

I would think that the right way to do this would be to get all the parts for a stock rear disk setup (master cylinder, bias valve, etc).

[glnr13]

i'm running disc conversion with original drum MC. I autox the car and have not found it act or function strangely or poorly.

Yes, you are changing the system but the rubber hydraulic lines are in the same position and so is the junction. the function of the system is the same: step on brakes, fluid goes to calipers, calipers clamp down.

You should do a full flush since you're in there.

[E30SPDFRK]

I would think that the right way to do this would be to get all the parts for a stock rear disk setup (master cylinder, bias valve, etc).

Would the abs influence the need for the different master sizes/proportioning valve? Thats one part of the system I won't be adding.

I can see the bias valve on realoem.com but the number is grayed out and I can't find it in any other system, is there a difference between disk and drum ones or are you saying to replace it just to have a fully new system?

i'm running disc conversion with original drum MC. I autox the car and have not found it act or function strangely or poorly.

Yes, you are changing the system but the rubber hydraulic lines are in the same position and so is the junction. the function of the system is the same: step on brakes, fluid goes to calipers, calipers clamp down.

You should do a full flush since you're in there.

The function is the same, but that doesn't mean they are the same. A wheel cylinder uses pressure to operate the brakes the same as a piston would in a caliper, but it uses a different amount of fluid to do it and might respond differently to different pressures. A difference in master cylinder sizes affects braking modulation and pedal feel. It's not as simple as it might seem.

Since you have experience with the smaller master and disks, have you ever had the chance to hop out of your car and into a factory 4 wheel disk car to compare them?

[glnr13]

Would the abs influence the need for the different master sizes/proportioning valve? Thats one part of the system I won't be adding.

I can see the bias valve on realoem.com but the number is grayed out and I can't find it in any other system, is there a difference between disk and drum ones or are you saying to replace it just to have a fully new system?

The function is the same, but that doesn't mean they are the same. A wheel cylinder uses pressure to operate the brakes the same as a piston would in a caliper, but it uses a different amount of fluid to do it and might respond differently to different pressures. A difference in master cylinder sizes affects braking modulation and pedal feel. It's not as simple as it might seem.

Since you have experience with the smaller master and disks, have you ever had the chance to hop out of your car and into a factory 4 wheel disk car to compare them?

Sorry for the late response.
I have had a chance to drive a car that was all original disk and it felt the same from my point of view.

Sent from my SAMSUNG-SM-G900A using Tapatalk