I’ll answer your questions in order.
First, why the too-large MC is negatively impacting your front braking. This is not necessarily intuitive so most people don’t understand this. I will explain it using Newtonian physics instead of fluid mechanics (it’s just easier to understand that way). Start with two facts: Work equals Force multiplied by Distance and the Work at the MC equals the Work at the caliper pistons (ignore the expansion in your brake lines for simplification). Your MC piston travels a longer distance than your caliper pistons do, so the force output at your caliper pistons must be greater than the force input at your MC. The difference in the bores and the distances traveled means that the force from your leg is far less than the force of the caliper pistons on the rotor. I’m excluding the force multiplication from the pedal ratio and the addition of force from the booster but you get the idea because regardless of the size of your MC and caliper pistons, the contribution of those to the force on the MC stays the same. That’s the basics of how brakes work.
So, all other things being equal, let’s look at your situation. You have a super stiff pedal that you don’t need to press very far on to get “full braking”. Let’s call that distance D1. As a result, you get a force at your caliper piston. Let’s call that force F1. Now switch to a smaller bore MC. Your new MC (and your pedal) will travel a longer distance before “full braking”. Let’s call that distance D2 (which is greater than D1). You are using the same pedal ratio and booster in both cases (and your leg doesn’t get any stronger) so the force on the MC stays the same. From the perspective of Work, the Work from the smaller bore MC must be greater than the Work from the larger bore MC because D2>D1 and the Force input to the MC in both cases remains the same. So Work(MC) given D2 must be greater than Work(MC) given D1. Move now to the caliper. Remember that Work(MC)=Work(calipers). Since Work(MC) has increased from a smaller bore MC, than Work(calipers) must have increased the same amount. The distance that the caliper piston travels does not change, therefore the new force at the caliper (let’s call that F2) must have gone up. So F2>F1.
In short, a smaller bore MC will create more force at the caliper than a larger bore MC. All other things equal, if you get more pedal travel you get more brake torque. Of course you don’t want to hit your floorboard, so you have to get a big enough bore to keep that from happening, but you want to get as much pedal travel as possible to get the maximum brake torque you can get. Ergo, your too-large MC is robbing your front calipers of potential force.
I just went through the same thing. I was using T-bird brakes at the front and was very happy with my 1.25” MC. I switched to our Huge Brakes kit up front which has more surface area on the pistons. I erroneously assumed that the larger bore pistons would require at least the same if not more fluid to reach the rotor. I was wrong (I guess modern caliper pistons don’t need to travel as far as 45 year old ones do). I had very little pedal travel AKA a “super stiff” pedal. I just switched to a 1.125” MC. My pedal now travels further but I am getting more brake torque up front as a result.
Now to the points about your prop valve, I’m not suggesting it’s broken, I’m just saying that it’s useless given your current setup. The problem is that the amount of line pressure that locks your rear wheels is less than the point that the prop valve limits to. Once your tires lock up, additional pressure isn’t going to lock them more. Once they are locked, they are locked. Your prop valve isn’t stopping your rear tires from locking so it’s useless to you right now. If you had smaller brakes at the rear that took more pressure to lock the tires, your prop valve would be fine, so you don’t need to toss it or replace, you just need smaller brakes in the back. Then your prop valve will serve its purpose.
If it were my rig, here’s what I would do. I would put the Caddy calipers on the rear and see what happens. Test to see if the prop valve works in that situation and can it keep my rear tires from locking up. Test to see if I like the brake bias in general. If yes, and my pedal is stiff, I’d try out the Chevy MC with some adapters and see how deep into my brake pedal I get. Brakes are a critical safety system and I’d spend the time to test out these basic fixes, at least. That’s just me; it’s your rig and it’s your call.
I know this is long and somewhat confusing. Feel free to hit me up on email:
chad@hugebrakes.com. I’m happy to help if I can.
