I recently installed the toyota 4 piston brakes (the ones that go with the vented rotors) The stock master cylinder (7/8") was very spongy, so I swapped it out with a 1" bore master cylinder. With this setup, the brake pedal has zero resistance for half of the travel. In this half of the travel, 90% of the braking force is applied. This gives it a very touchy feel. Has anyone else had this problem before? What master cylinder should I use for this setup?

 

msa recommends that you install an adjustable proportioning valve for this swap. Your 7/8 master is big enough.

 

I could not see on the msa website anywhere that they recommend an adjustable proportioning valve for this setup. This is the same exact setup that they sell without the adjustable valve. I have read on this site that many people have upgraded to a larger bore mc since their 7/8" one did not work. Any more ideas?

 

I have the larger S13 brakes on mine with rear discs. I'm using the stock 7/8 master without a proportioning valve and they work fantastic although I think I might try one. The 4 piston calipers have double the grabbing power of the little stock ones. You have even less in the rear drum type. By increasing the master you give the whole system more pressure. I think you need to balance the system. You could call tech support at msa.

 

Here is an update on the problems. I changed the stock 7/8 mc to a larger 15/16 mc. This fixed the problem with the brakes not applying, but caused just an opposite problem. The brakes are super touchy with little to no effort to push the pedal for half of the pedal travel. The second half of the pedal travel is rock hard with an abrupt transistion between super touchy and super hard. The rear brakes did not work as well. I removed the proportioning valve and got the back brakes to work. I thought that increasing the mc size would increase pedal resistance. I changed the 15/16 mc to a 1-1/32 mc. This did not fix the touchy pedal feel, but did bring the transition between super touchy and rock hard a little closer to the top of the travel. Has anyone heard of these problems??
-jeremy-

 

maybe your power brake unit is bad. What is the casting number on the calipers, which rotors are you using? Was everything a direct bolt on or did you have to shim anything?

 

Another thought- If your calipers are meant for vented rotors then your calipers won't squeeze down far enough. The pistons will bottom out. This may explain why you have sensitive pedal part way down and then it gets hard. You could try removing one caliper and step on the brake. It should close down past the thickness of the rotor.

 

I do have vented rotors. To get the rotor to fit in the caliper, the pistons had to be all the way into their bores.
-jeremy-

 

The only other thing I can think of is maybe your master cyclinder is bottoming out. I didn't change mine so I don't know if you need to change the push rod or the linkage.

 

I did have to change the pushrod length for each of the master cylinerders that I tried. It isn't bottoming out though. When the pedal goes from easy to push to hard to push, it still provides more braking when I push harder. I am more concerned about how easy it is to push on the top half of the travel. The brake pedal for the top half of the travel is easier to push than my gas pedal! I am thinking a quick take up mc might fix this problem.
-jeremy-

 

There is a spacer between the piston rod of the mastercylinder and the vacuum assist diaphragm. If this drops when you change out the mastercylinder, you will get the exact symptoms you are describing. Check a service manual for a drawing of the vacuum booster/mastercylinder assembly, and you will see the part I mean. I would post it, but I don't have access to my manuals right now. Then check to see if the piece has dropped in your unit.