Disc brake issues (Page 1 of 2)

I half bench bled the MC by attaching the bench bleed kit to the MC after it was already bolted to the firewall.  Figured it was the same in the end.  Push it with a screwdriver or push it with the brake pedal.  Same outcome.  We just finished bleeding the brake system for the second time with the "furthest first" system.  Same pedal feel.

Agree, pressure bleeders are amazingly quick and effective.  Dont apply too much pressure as the cap seals can fail?

If you are sure there is no air left in the system, try clamping off the brake hoses and then see how the peddle reacts. Should be like pushing on a brick. Then take clamps off one at a time until the culprit rears its ugly head.(I have seen calipers retract too far causing the same symptoms) If the peddle is still reacting the same way then I would suspect the master cylinder. Good luck, hope this helps.  Dave

If you stab the pedal hard the first time, does it stay up?

If the pedal is slowly, lightly pressed will it go to the floor?

If the answers above are yes, then a tired MC is the culprit.

Wow. Thanks for all your quick replies. Ive been ripping my hair out this week trying to just get it to start with one issue after another. Everyone on the team is eager to help but most don't really "get" automotive systems and we don't have decades of experience.  I am the best at most of it so if something stumps me I have no one to spitball with.  I'm glad there's a forum like this for my dumbass questions. You guys are lifesavers. Come on by Friday.  If you let us pick your brains we'll feed you BBQ.

No brake booster by the way. We have manual flavor brakes

Spent weeks getting our front disc conversion to behave.

Is yours a single or dual circuit system?  If single, like ours, then adjustment of rear drums will be critical to this process. Make sure rears are set properly, as that will affect the whole magilla. You're working in the range of ten-thousandths.

Our rears are not self adjusting, so, with wheels bolted firmly on - jack the rear, have an assistant pump and hold pedal pressure to center the rear shoes in the drum. Adjust firmly until tight, then back off slightly. Release pedal, check for faint resistance when spinning the wheel. Do again for other side. That should have rears adjusted and thus good pressure in the entire system now.

One more thing - might need a larger diameter master cylinder - we did when we went from rear drums to rear discs. Sounds like the symptom we had.

This is likely the cause.
Disk brake calipers have a bigger piston and so more fluid has to flow. A small MC will mean lots of pedal travel but better feel.

A couple items you should probably check.
A) did you rebuild the calipers or just slap them on from the JY? They could be old and kind of rusty/sticky. Strip the pistons out and clean all the seals, the bores, and the sides of the pistons. A scotch-bright pad works pretty good for the last two. I lube up the seals with STP son of a gun, its pure silicone and will help preserve the seals, and make the pistons slide in and out nicely.

B)You might also check on the MC from a similar car with disk brakes to see if there are check valves (these could be just rubber disks on the MC outputs), some cars have these to prevent the return flow of lots of fluid and keep the caliper pistons/pads close to the rotor.

C) See if you can find out if there is a similar bolt pattern MC in a another model of that manufacturer that has a bigger bore. Datsun cars are great for this, lots of choices in the same bolt pattern (510, 240z, 280z, 200sx). Bolt a bigger one in place to get more volume to the front calipers. But this will probably over power the rear drum brakes.
So there are two basic fixes for that. Run great pads up front and cheap pads in the rear to get some sort of F/R balance, or buy a proportioning valve and install it in the line to the rear brakes. If you are really smart you'll put it next to the drivers seat so your drivers can mess with it during the race (adjusting for the change in fuel load), which will result in the brake bias being all F-ed up when the next guy gets in the car

a TIP for bleeding the master. Take a couple short ~12" pieces of clear hose and put them on the bleed ports and then stick them in the fluid reservoirs, a short loop. open bleed ports Pump away and watch for clear lines, rap on the MC with a wrench a couple times - repeat.

Been down this road before.  Be 100% sure that all four calipers are square and not moving while being applied.  I had one flexing (street car, one bent caliper pin) that had exactly the same symptom.  when it would relax after a minute or so it would press lightly back on the piston.

Forgot about that one I've seen a bunch of calipers installed on the wrong side of the car-- bleeders pointed down. Should be pointed up.