Bias change with 25.4mm master cylinder.

[redcar]

Hi there, I've read that changing from a standard e30 brake M/C (22mm/18mm) to an e32 750i M/C (25mm/25mm) increases the rear bias when braking.

Please could someone explain this as it doesn't make sense to me. I thought increasing the size of the master cylinder bore would lead to a decrease in line pressure, therefore changing to a 25mm/25mm M/C would surely lead to a reduction in rear bias.

The reason I ask is I use a full Porsche Boxster brake setup, 4 pot front and rear with 300mm front disc and 294mm rear disc.
When I used the 25.4mm master cylinder I was having problems with the rear wheels locking under breaking. I'm looking for solutions to rectify this but I'm having trouble understanding the contradictory information on the Internet.

[maggspower]

Ah, the internet, full of internet experts and the home of internet bullsh1t

You are indeed correct that the 25/25mm MC will lower line pressure and lessen the pedal travel.

The Porsche rear brakes are big, and on the e30 the differential between front and rear is now very small. As with most Porsches, they tend to be weighty at the rear, so there is a bit better traction to keep the wheels stuck to the floor.

I take it this locking is on the limit? Are you running the e30 pressure reg for the rear brakes. Found in the engine bay, passenger side against the bulkhead?

The cure will be an inline bias valve, so that you can tune the rears.

[redcar]

Thank you for confirming. Everything on the Internet suggests that the 25/25 M/C will push more volume into the rear calipers as its a bigger bore and therefore lock them first, but this didn't make any sense to me as I thought the master cylinder would deliver equal line pressure front and rear and volume of fluid moved is irrelevant.

The rear calipers are actually a similar overall piston surface area to an e34 540i rear caliper so surprisingly they aren't as big as you'd think in relation to the front.

The locking isn't actually on the limit, it pretty much locks the rears very easily, impossible to lock the fronts and they're simply unusable and unsafe.

Forgot to mention that I have a Tilton adjustable proportioning valve and the standard e30 valve has been removed.
Even when this was set on the most reduction for rear line pressure the brakes were unusable.

I now run a pedal box with 0.625 front and 0.75 rear master cylinders with the proportioning valve set to a few clicks reduction and the brake balance is good at maximum braking force but I don't get on with the increased pedal force of the manual pedal box hence trying to go back to a servo setup.

[redcar]

Also forgot to say, does it sound plausible my master cylinder or adjustable Tilton proportioning valve may have been faulty?