I have a 70 Chevelle Malibu, and I just switched over from drum/drum to disc/drum. I have fluid to all four corners with solid flow, now sputtering or bubbles. and with the motor off the pedal feels decent with decent return. Problem is with the engine running the pedal goes straight to the floor and is super slow to return. I checked my vacuum from the back of the carb to the booster and it sits at 16"hg solid at idle.

is my soft pedal due to too little vacuum? or do I have a rod problem in the MC? Any help would be great


Thanks

New master cylinder?

Check out the rod first.

Thanks for the reply,

yes everything came new in the kit, MC booster, lines, etc. the MC had two different rods one short one long. I have tried them both. short rod didn't seem to move the piston much, as there were no bubbles or anything in the resevoir. went to the long rod and was able to bleed the brakes. Although the long rod doesn't seem to do much until you get to the end of the pedal. MC also came with a alum. spacer as well, and I have tried different configs. of spacer and rod length.

Did you bench bleed the master cylinder before installing it?

yeah I did but I only went until the bubbles stopped coming out. I read on here that I should have went until I could only push it it in a 1/4" or so. Is that correct?

yeah I did but I only went until the bubbles stopped coming out. I read on here that I should have went until I could only push it it in a 1/4" or so. Is that correct?

Ive done it to where the bubbles stopped. You'll still be able to push the piston in all the way.

I would start messing with the adj. rod.

I just wanted to add that I verified that I get the little eruptions when I push the pedal, haven't seen any little air bubbles or anything but I will need a helper to verify. I also started the car and pressed the pedal and again it went to the floor and the brake light came on.

I am guessing that I must have alot more air in there somewhere than I thought. My MC doesn't have any bleeder screws, should I take it off and re-bench bleed it. Thanks in advance--Mason

Ive done it to where the bubbles stopped. You'll still be able to push the piston in all the way.

I would start messing with the adj. rod.



The kit came with 2 rods and the MC came with a alum. spacer with a fairly shallow pocket. So I tried the shorter rod with the spacer and was unable to bleed the brakes. Switched to the longer rod with no spacer (how the old MC was set up) and was able to get where I am now. I also tried the long rod and the spacer and there was alot of pre-load when I went to tighten the MC down. I'll try throwing in the short rod and the spacer and see if that helps at all. Thanks--Mason

Did you change the position of the rod where it connects to the brake pedal. There are 2 holes One is for drum brakes and the other is for disc/drum.

Good Luck

thanks for the reply. Yeah I thought of that one late last night so I ran down to the garage and verified that it's in the lower of the two holes.

Ive done it to where the bubbles stopped. You'll still be able to push the piston in all the way.

I would start messing with the adj. rod.


I pulled the MC and put it back on the bench, capped everything off and pushed in a couple of times with a screwdriver. Got a couple of good sized bubbles to come out. I also reinstalled the alum. spacer in the MC and swapped in the shorter push rod. Feels good with the motor off lots tighter. I verified that I got the eruptions with the shorter rod in as well.

Fired up the engine and tested the brakes. Still feels pretty spongey, but it didn't go all the way to the floor, and the brake light didn't come on so I think there's been progress. Thanks Joe!

I am going to gravity bleed one more time and see if that helps at all.

Gravity bled one more time, and switched to the short pushrod with the spacer in the MC and everything panned out great. I got a alignment on it yesterday and spent the rest of the day cruising around. Thanks for the help