Brake bias and master cylinder selection

[Javal]

Hi Team,

Let's talk about brake master cylinder sizing.

My current situation is this - I have an upgraded and RT132 Corona braking system, fitted to my XT130. I've replaced the standard Sumitomo twin pistons with LN106 Hilux 4 piston calipers (the old C/P/H upgrade) and the T-series Drum braked rear end with a disc-converted T-series diff, running ST141 Corona rear calipers and discs.

Now I am having some dilemmas with bias and master cylinder choice

BIAS

Std Sumitomos - 50mm pistons = 1963.49 mm2 x 4 pistons on the front = 7853.96 mm2 of total front piston surface area
LN106 Hilux - 40mm pistons = 1256.63 mm2 x 8 pistons on the front = 10053.04 mm2 of total front piston surface area

This is an increase of 27.9% over standard.

The rear end has a much more stark increase.

Std Wheel Cylinders - 20.22mm = 321.10 mm2 x 4 pistons on the rear = 1284.40 mm2 of total rear piston surface area
ST141 Calipers - 38mm = 1134.11 mm2 x 2 pistons on the rear = 2268.22 mm2 of total rear piston surface area

This is an increase of 76.5% over standard.

Previously my front to rear surface area ratio was - Front 6.11mm2 : 1 mm2 rear
It will be, with the upgraded components - Front 4.43 mm2 : 1 mm2 rear

After all this calculation, all i've concluded is that my front to rear bias is now going to be up the shit, but I thought i'd put it in the body of the post so if anyone actually knows what they're talking about and i've done this all completely wrong, it's plain for all to see.

MASTER CYLINDER SIZING

As for master cylinder sizing. The XT130 has a 7/8" master cylinder (22.22mm, giving it an area of 387.77 mm2), meaning there is 23.56 mm2 of slave (pistons) area for every 1 mm2 of master piston area. (cumulative area of old slaves divided by master area)

To keep this ratio correct, I need a master cylinder with a surface area of 522.97 mm2. (cumulative area of upgraded slaves divided by 23.56) The closest I'm going to get to his is a 1" master cylinder (giving me 506.70 mm2).



Now that all of that is calculated and we have all the facts in front of us - question time!

I WAS going to use an RT132 Corona master cylinder (this is a 4-bolt design and also 7/8"), is it possible to have it bored out to 1"?

Alternatively, I suppose the MA70 master cylinder could be used as per the C/P/H thread, however I'm unsure whether it will fit on to the RT132 booster, and I am very reluctant to change boosters. As mentioned before, it's a 4-bolt, and the MA70 appears to be a 4 bolt. Does anyone know the bolt spacing?

Now even IF i can go through with either of these options, my bias is still going to be up the shit. Now, the RT132 runs a proportioning valve, just below the master, I assume to allow the correct pressures with the original disc / drum combination. If I were to gut this proportioning valve, in combination with an upgraded master / MA70 master, would this be suitable for my hilux + disc / corona + disc combination.

Please help. I'm kinda wallowing in a sea of not-knowing-enough-to-make-an-informed-decision on this one.

[crowncustom]

A 15/16" from either a MX 73 Cressida or MX 10-13 Mark 2 Corona should work quite fine both 4 bolt.I've done the same conversion on my MX 22 and kept the original setup but the difference with what you have is that I'm still running the standard proportioning valve which only runs off the rear line where you have the valve to proportion front to rear.I have tested the brakes only in my driveway as the car is still undergoing body repairs and such and I have noticed that the right rear locks when the brakes are applied a little harder than normal so this means to me that the valve that I have needs to be either replaced with the type that you have or modify mine to be able to adjust it.
Hope this sheds a bit more light on your conversion.
Cheers Brett.

[Javal]

Thanks Brett! It's useful info but considering the real-world (on road) behavior of the system isn't known yet and you're still running a disc / drum proportioning valve, i'd rather work this out on the maths and theory than use speculation as the base of my master cylinder choice.

Anyway, let's simplify the question.

If I fit an MA70 master cylinder (which is set up for disc/disc) and either delete or somehow 'lock' my proportioning valve (not gut it, as that would allow fluid to pass from one circuit to the other, giving 50:50 bias) would this solve my master cylinder problem?

[petergoudie]

You are ignoring another dimension and that is stroke. You also need to consider the stroke of the master cylinder and caliper pistons.

Have you considered an adjustable proportioning valve and then visit a workshop with a roller brake dynamometer? This will allow you to adjust the brake balence while the wheels are turning.

[BrianGT]

I would fit an adjustable brake balancing valve instead of the fixed valve you have at the moment.

It will definitely allow you to bias the brakes to suit your driving style. I used one for rallying and proportioned more to the rear so that the rear brakes locked very slightly before the front...great for rallying but maybe not for the road.

You might be surprised at how little bias the std valve allows to the rear and your rear brake upgrade might not be quite so catastrophic as you think.