Topic: Brake Problem

[Chevota22]

OK... I have an 86 yota pickup that I can normally figure out when I have problems, but I need some of the experts on the Marlin forumto help me out.....Please. It has an SAS, 5.29's, Aussies, 56" rear springs Chevy 4.3, 4L60E tranny blah blah blah. My issue is with the brakes. I did a rear disc brake swap with monte carlo calipers and 1/2 ton chevy rotors. Everything went together fine. I finished the install, bled the brakes, and had a nice pedal with the truck not running. If I start the truck up, my pedal goes almost to the floor and my brakes will barely stop me. The brakes were pretty bad before because of a leaking wheel cylinder on the old drum setup. Why is this happening? Oh, I forgot to mention that I upgraded to a newer master cylinder with the 1" bore.

[Burl]

What front brakes are you running I just did the same thing with my 85 runner upgraded to a 1 inch bore mc and front brakes didn't work for then I upgraded to the v6 calipers and vented rotors now it works really good

[Chevota22 [OP]]

V6 rotors as well if my memory is not too far off. I did that when I did my SAS I'm pretty sure. I just can't figure out why I lose my pedal when I start the truck up... I wish I knew because I can't even drive the thing. I'm almost thinking somethings up with the brake booster hooked up to the 4.3.

[Burl]

could be but check that you do have the v6 calipers because the 4cyl one are not very compatable.

[Chevota22 [OP]]

How do I tell for sure which ones I have. I know that they are the 4 piston design. Also, do you mean "the 4cyl one are not very compatable" due to the MC swap? I am just wondering how this could make my pedal sink.

[Burl]

I think it has to do with the volume of fluid available to be forced into the smaller calipers the 1 inch bore is more volume than the stock one's.  There might be some air still trapped in the system.  Did you install a porportioning valve to divert 60% to the front and 40% to the rear or a manual adjustment one that you can set to your preference

[79coyotefrg]

sounds like maybe the brake booster is bad or the master isnt bled completely

[Chevota22 [OP]]

I bench bled the MC, but is there a way to bleed it once installed to make sure I got all of the air out? Is there some trick with the rear monte carlo calipers with bleeding that I dont know about? I just used my vacuum bleeder. I haven't installed a proportioning valve yet.... thats the next step after I get this problem taken care of.

[Burl]

you should be able the open the bleeder valves individually starting with the furthest from the mc and attach a clear fuel line to save the fluid from getting all over the place.  let each valve drain for a while making shure not to let the mc get too low.  Once this is done you could prob rule out air in the lines.

[emsvitil]

It's about ratios.........  (caliper piston area to MC piston area)

The 86 front caliper has 4 pistons;   2 @ 1.335" diameter , 2 @ 1.685" diameter

So each caliper has a piston area of (I'm not showing the math....) 7.26 sqin, or 14.52 sqin total front.

The master is 13/16" for an area of .52 sqin.  

So you have a hydraulic advantage of 14.52/.52 = 27.92

Just changing the master to 1" (area .79 sqin) lowers your hydraulic advantage to 14.52/.79 = 18.38

So you have to press (27.92 / 18.38 =) 1.52 times harder at the pedal to get the same braking.

BTW,  the later V6 calipers have 4 1.684" diameter pistons for 8.91sqin or 17.82sqin for the 2 calipers.   To get the same 28:1 advantage, you'd need a .9" diameter master.  (between 7/8 & 15/16)

[Chevota22 [OP]]

how could I tell if my brake booster was bad? Also, I had this same problem when I had the stock MC on there.

[emsvitil]

how could I tell if my brake booster was bad? Also, I had this same problem when I had the stock MC on there.

Engine off:

Press brake pedal about 5 times to make sure the booster is depleted.

Hold pedal down, and while holding it down with constant pressure, start engine.   
If it's good, the pedal will go farther down.

[Chevota22 [OP]]

Well then I guess its good, but going down seems like a bad thing to me. The pedal goes to the floor. Maybe I'm just confused here, what am I missing? How do I get a firm pedal AFTER the truck is on???