Can you run 4 brake calipers with the same size caliper piston bore (size 2.80)?We have power bled and gravity bled these brakes numerous times and still get a soft pedal. After you pump the pedal 2 times, we have a firm pedal. Do we still have air in the lines, or are we doing something wrong?We have a 78 streetstock Camaro. We have put in a new corvette manual master cylinder, new 3/16 brake lines, and 4 new brake calipers with piston bore size 2.80.
first answer.. well.. your brake bias will be WAY off.. what.. i am NOT a brake professional.. only the answer man around the forums here. and i try my best.. brake bias on 4 wheel disc brake production cars is usually about 2/3rds on the front brakes and 1/3 on the rears.. this is for cars with equal sized tires on all 4 corners.. as when you apply the brakes. the weight shifts forward onto the front brakes.. but it reduces loading on the rear tires.. so less braking forces are needed to the rears.. with disc drum set ups.. there is a way to change the wheel cylinder size to increase by going to larger wheel cylinder or decrease by going to smaller wheel cylinders. so you can get the wheels to lock up all at the same time. with disc brake piston size all the same.. i am thinking that you will need to go to one of the double master cylinder setups.. where you can turn the knob on the dash.. to change the fulcrum point where the brake pedal pushes on the spreader bar to the twin masters.. if you shift it in one direction.. you get more pressure applied on one master.. less on the other.. and visa versa.. http://www.wilwood.com/m/techtip/TechPedalTip.aspxon the soft pedal.. a few thoughts from a mechanic. ME.. what kinds of pads.. are the rotors machined properly.. not worn .. have you done any driving of the car to bed the pads in.. so the surface of the brake pad is actually worn in slightly removes the spongyness from the surface. allows a nice firm grip on the pedal instead of the fuzz on the face of the new friction surface. check with the manufacturers about bedding that exact formula of brake pad material in.please.. jack up your car.. so you can remove all 4 tires.. if you are using stock type rotors on the front.. great..i don't know what kind of rotors, caliper brackets and caliper type you have one the back.. please.. take some flat washers.. stick them on the studs them put the lug nuts back on.. this holds the rotors to the axle flange and duplicates the effect of having your wheels bolted on.. have somebody step on and release the brake pedal while you examine the operation of the 4 individual brake calipers.. this is engine off.. transmission in neutral.. so you are able to spin the rear end by hand when the pedal is released.. watch the calipers apply.. you should see the inner pad and piston extend against the inside of the rotor. then the caliper slide ever so slightly on the caliper mount bushings so the outer pad applies with an even braking force ...there should be NO FLEXING of the caliper bracket. when the pedal is released.. look carefully. the inner pad should retract just slightly.. the axle should be free to turn.. and again.. the CALIPER SUPPORT BRACKET SHOULD NOT DEFLECT.. when you have the car up in the air.. take some images of the brakes.. 45 degrees to the side of the caliper so we can see the mounting.. and also straight in line with the caliper.. so we can look down thru the caliper and into the vent holes in the rotor.. if you have a cell phone camera that does video.. record the apply and release of each caliper a few times on each caliper. not just a single apply.. record a minute of apply and releases.. then post all 4 to youtube.. and link it here.. i will look and see if i can spot any issues.. back to the twin master cylinders.. hmm.. could you mount the twin masters to the face of the firewall where the power brake booster would mount. so the push rod would activate the adjustable bias bar to change the apply pressure to the individual masters.. i don't know.. i just thought of it. i bet that someone will sell a bunch of those.. could you stagger the individual master sizes. yep.. lets do a little more diagnostic. if you don't get caliper bracket flexing.. or caliper slides that DON'T move freely..break out your dial caliper.. or just a decent ruler..pop the mounting bolts on a caliper.. pull the caliper off and flip it upside down.. retract the piston fully.. slip the pads in.. measure the distance between the pads.. then.. measure half the thickness of each pads friction material.. do not include the pad backing plate.. add this up.. distance between the pads plus half the thickness of each pad.. cut a block of wood that thick.. so you can apply the brakes with the caliper off.. or the space between the pads and half the thickness of each pad.. this is because i want you to remove ONE caliper at a time and have the helper apply the brakes to extend the piston without popping it out of the seal.. i would like you to then fully retract the piston. while holding the open face of the piston straight down. this will force any air trapped in the caliper back out the brake line and hopefully all the way to the master cylinder where it might have a chance of coming out of the fill passages in the master cylinder bore.. i learned this one day.. when the brake caliper screw broke off flush with the caliper surface and there were no calipers available and i needed to bleed that caliper slightly.. the fluid comes in below the top of the piston bore. the bleeder screw passage allows air out of the top of the caliper bore when the calipers are installed at the proper angle.. with the bleeder screws on the upside of the caliper.
in thinking.. you might perform some research depending on what rear calipers and rear brackets you have.. to find some with less bore size that you can swap on. if you find out you have too much rear brake.. and the rears lock up before the fronts.. you need to try them in a safe location..look at the last few seconds of this 27 second video... note the rocking of the caliper as the brakes are applied and released.. the caliper bracket is either not parallel or the spacing is wrong .. http://www.youtube.com/watch?v=l4wamopyEsUthe caliper rocking will cause everybody spongy brakes..
Thanks for the info Wayne,Our rear calipers are indeed flexing when we apply the brakes. We are planning on welding a bracket for support. We acquired this problem when we purchased the car and are starting to figure it out in between races.What do you think about the smaller bore caliper for the rear?
The rear calipers are mounted at 12 o clock on the rotor and they're laying on there sides. Is it proper to hang them on wire when bleeding?
pictures... videos.. let me see before you do any welding to firm things up.....use photobucket to post your pictures.. paste links here..use youtube to post video.. pull the caliper mounting bolts out and so they are not clamped tight to the bracket.. and see if they still flex.. i have been on various boards here since 2008.. i am really really great at diagnosing issues if i can see them.. if the calipers are FLEXING THERE IS AN ALIGNMENT ISSUE.. either angle or spacing.. if you strengthen the caliper brackets or weld the calipers.. you will snap the disc on the rotors..
I will take some pictures/videos this Saturday of the calipers when we bleed it. We have bled them hanging on a wire with a wood block to stop the pistons from popping out, still a soft pedal.
i will look forward to the photos.. perhaps the videos???things could always be worse..if the calipers are flexing.. after bleeding. you are still going to have a soft pedal.
Wayne, sorry for not posting the pictures or videos.Saturday afternoon we were going to put on the smaller piston calipers for the rear but we decided before we did that, to replace the brake line to the rear again. After we replaced the brake line, the soft pedal went away. The brake line we replaced had a crack in the double flare, it wasn't leaking, but I think it was still sucking air.