Bad master cylinder, or booster?

[Svashtar]

Car has been stopping fine since I did the brake job a few months ago (new rotors and pads and bled the system) but maybe I just got used to the spongy pedal. Even had one emergency stop with no problem when someone cut me off.

I noticed the brake fluid reservoir was down maybe 3/16” this morning, but no sign of leaks that I can see, so topped it off.

With the car off I pumped up the brake pedal and it was super hard. Turned the car on, pressed hard and the pedal slowly sinks towards the floor. Not a lot, but 2-3 inches.

That sounds like air in the lines, but I was careful about that when bleeding them, and never let the master cylinder get empty.

It seems to me like it might be the brake booster not holding pressure. (?)

I’ll be doing big brakes and SS lines in a few months, but don’t want to install all that on top of a core problem.

Am I right in thinking pedal pressure should be the same whether car is running or not? Thanks.


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[fastblackmerc]

Car has been stopping fine since I did the brake job a few months ago (new rotors and pads and bled the system) but maybe I just got used to the spongy pedal. Even had one emergency stop with no problem when someone cut me off.

I noticed the brake fluid reservoir was down maybe 3/16” this morning, but no sign of leaks that I can see, so topped it off.

With the car off I pumped up the brake pedal and it was super hard. Turned the car on, pressed hard and the pedal slowly sinks towards the floor. Not a lot, but 2-3 inches.

That sounds like air in the lines, but I was careful about that when bleeding them, and never let the master cylinder get empty.

It seems to me like it might be the brake booster not holding pressure. (?)

I’ll be doing big brakes and SS lines in a few months, but don’t want to install all that on top of a core problem.

Am I right in thinking pedal pressure should be the same whether car is running or not? Thanks.


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Pedal feel will be different with the engine off or with it on. That's why we have power brakes.

I'd go ahead and replace the lines with the SS ones. I know mine was night and day after I did it.

[Svashtar]

Ok thanks. I’m looking at two different big brake setups now, but no matter what will also install the SS lines front and back. I’ll get a decent pressure bleeder as well. Last time I did the one man bottle and tubing thing, which worked fine, just slow.


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[fastblackmerc]

I gravity bled mine.

[Turbov6Bryan]

Ok thanks. I’m looking at two different big brake setups now, but no matter what will also install the SS lines front and back. I’ll get a decent pressure bleeder as well. Last time I did the one man bottle and tubing thing, which worked fine, just slow.


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If you don't have a little buddy helper, use this style. No air will get back in the line..
Have you thought of the Zack big brake upgrade?

Not this size fitting, just a general description

http://www.jegs.com/i/Russell/799/639570/10002/-1?CAWELAID=1710529761&CAGPSPN=pla&CAAGID=44693592161&CATCI=pla-210001117271&CATARGETID=230006180039217681&cadevice=m&gclid=EAIaIQobChMIgfCZsv6M2AIV jlcNCh0eJQksEAQYBSABEgJgIfD_Bw E

[Svashtar]

I gravity bled mine.


I tried that and just couldn’t get any volume going. The fluid just sat in the tube. But come to think of it that might be because you have the speed bleeder valves.

I did the soda bottle filled with 2” of clean brake fluid, with a hole in the cap, with a tight fitting hose running from the bleeder valve down into the bottom of the bottle. I parked it on an upended bucket, then went around and _gently_ pumped the brakes a few times, until I got clear fluid in the lines. Worked fine, just slow. (The 14 year old fluid pumped out looked like green ink.)

With no brake fluid at the bottom of the bottle and the speed bleeders, gravity would have probably worked as well.


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[Svashtar]

If you don't have a little buddy helper, use this style. No air will get back in the line..

Have you thought of the Zack big brake upgrade?



Not this size fitting, just a general description



http://www.jegs.com/i/Russell/799/639570/10002/-1?CAWELAID=1710529761&CAGPSPN=pla&CAAGID=44693592161&CATCI=pla-210001117271&CATARGETID=230006180039217681&cadevice=m&gclid=EAIaIQobChMIgfCZsv6M2AIV jlcNCh0eJQksEAQYBSABEgJgIfD_Bw E


Thanks, I appreciate the link! Fastblackmerc mentioned those several months ago, and they’re on my list. Helpful upgrade.

I’m looking at all the brake kits, Zacks, the Corvette kit, TCE Aero6, but still gathering info. I have a favorite, but don’t want to spend more than I have to, and a couple of guys have been really helpful with PM’s explaining their setups.


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[Turbov6Bryan]

Buying brembo rotors gets expensive, that's what is great about zacks kit.

[RubberCtyRauder]

the TCE SS lines are not the same for TCE kit and Zack kit. , Zack kit is the corvette caliper.

[BAD MERC]

I gravity bled mine.

That's not a common term. I did mine the same way. The fluid level must be maintained in the master but it's effective. This is the ideal way to singlehandedly replace a caliper, too.