Is my master cylinder dead?

[MW93cb7]

Hello all, I just got through bleeding my brakes and there was a ton of air in the system because the master cylinder was emptied. Anyway, as I was going to pull it out of the garage the brake pedal went to the floor with no resistance. There are no puddles anywhere and the calipers are brand new. Pretty sure I got all the air out of the system, and if not there would still be some pedal resistance right? Is my master cylinder dead, or did I not get all the air out?

Thanks

[dj_ender]

Sounds like you may have not bled the master cylinder properly prior to bleeding the system. When this happens it either becomes damn near impossible to bled the system, or it can seem bled for a bit and then the air bubble will show its head and allow your pedal to fall to the floor. I suggest removing the brake lines to the master cylinder and re bench bleeding the cylinder, then re bled the whole system. If the problem persists it could point to a leak or a bad cylinder. Hope this helps

[DIRTY DAVE]

^^^^agreed with.

[MW93cb7]

You are absolutely right.

I actually thought for a second about my question after I posted and feel like an idiot, cylinder cant be dead because it was just pushing the fluid out my bleeders.

I completely forgot that I had to bench bleed the cylinder since all the fluid left the system. Usually have some fluid left in the reservoir when I bleed them, so I never even thought about it. Feel retarded for wasting an hour. Thanks a lot for the reminder. Guess I will hop on that tomorrow, it's always the simple things.

[batever]

you don't need to remove it from the car to "bench bleed" it. Just get one of those $10 self-bleeding kits with the tube and the bottle, unhook the lines from the master cylinder, level the master cylinder (it's pretty level anyways, but completely level it) by lifting up the rear of the vehicle), and bleed it like that right while it's on the firewall before reattaching the lines.

[dj_ender]

No problem, hope you get it fixed soon. I let it go forever until one day I couldn't take it anymore and changed it out.

[MW93cb7]

Took It out and bench bled it. Hooked everything back up and same shit. Maybe the M/C is dead. Gonna pick a up a new one and put it in tomorrow and see what happens. If that doesn't work I am guessing it's something at the calipers since they are from a Prelude. I did the 11.1 in. ROH conversion.

It all seems to be bolted up properly and there are no leaks. Plenty of people have done this with no issue. So if it's not the 20 year old M/C it has to be at the calipers. This all looked so easy in the DIY lol

[dj_ender]

Oh snap! Did you just say that the original master cylinder was still on there? Yeah replace that paper weight, every cb I've ever owned has slowly had the pressure go out of the cylinder, requiring a change out. It's a common problem.

[MW93cb7]

Alright, so I replaced the master cylinder, and bled the lines. Zero air in system. Have perfect pressure in pedal when car is off. When I turn the car on however, I lose all brake pressure, and pedal sinks to floor.

Is my brake booster the one that is dead? If so, how much of a PITA is it to replace? (doesn't seem so bad, but bolts are behind the dash)

Pretty sure it has to be the booster because I have replaced everything else, short of the brake lines and there are no leaks. Looking for a second opinion before I spend more money.

Car has 276,000 miles, so I wouldn't be surprised.

[dj_ender]

Alright, so I replaced the master cylinder, and bled the lines. Zero air in system. Have perfect pressure in pedal when car is off. When I turn the car on however, I lose all brake pressure, and pedal sinks to floor.

Is my brake booster the one that is dead? If so, how much of a PITA is it to replace? (doesn't seem so bad, but bolts are behind the dash)

Pretty sure it has to be the booster because I have replaced everything else, short of the brake lines and there are no leaks. Looking for a second opinion before I spend more money.

Car has 276,000 miles, so I wouldn't be surprised.

Dumb question but, did you bleed them properly? By that I mean starting with the rear passenger wheel then moving to the rear drivers wheel and then front passenger, then finally the front drivers wheel. And did you do all of the bleeding with the MC fluid cap off? Just trying to cover all he bases before you go and purchase a new brake booster. I have personally never heard of anyone having to replace one, I don't think they go bad very often if at all. Now first let me say, I AM NOT FOR SURE ON THIS, but have you checked the brake booster line that goes to the IM, are there any leaks? I could be wrong but I think that could cause some problems. Other than that I am at a loss of ideas for ya

[batever]

Alright, so I replaced the master cylinder, and bled the lines. Zero air in system. Have perfect pressure in pedal when car is off. When I turn the car on however, I lose all brake pressure, and pedal sinks to floor.

Is my brake booster the one that is dead? If so, how much of a PITA is it to replace? (doesn't seem so bad, but bolts are behind the dash)

Pretty sure it has to be the booster because I have replaced everything else, short of the brake lines and there are no leaks. Looking for a second opinion before I spend more money.

Car has 276,000 miles, so I wouldn't be surprised.

I'm not sure. My instinct would be that a totally failed brake booster would result in pedal pressure and feel that is equal to when the car is off, not loose pedal pressure.

What do you mean, you lose all brake pressure when you turn the car on? It would be normal for the brake pedal to sink further once you start the car (with your foot already on the brake pedal) because the booster is using vacuum to add pressure to the master cylinder pushrod in addition to the power you are adding with your foot. that extra force is what really applies a large amount of the hydraulic force to the cylinders (and, in the process, causes the brake pedal to drop lower when you start the car).

Now, if the brakes are not engaging when this happens, then the problem is still in the hydraulic lines in my opinion, in the master cylinder, the hydraulic lines, or in a leak at one or several of the brake units (drum cylinders or calipers).


I could be wrong, there may be a way that the brake booster might cause such a symptom but I don't see how.

Better wait for others to chime in or start digging in a manual about the booster.

[ssondubs]

Alright, so I replaced the master cylinder, and bled the lines. Zero air in system. Have perfect pressure in pedal when car is off. When I turn the car on however, I lose all brake pressure, and pedal sinks to floor.

Is my brake booster the one that is dead? If so, how much of a PITA is it to replace? (doesn't seem so bad, but bolts are behind the dash)

Pretty sure it has to be the booster because I have replaced everything else, short of the brake lines and there are no leaks. Looking for a second opinion before I spend more money.

Car has 276,000 miles, so I wouldn't be surprised.

Did you get it fixed?

I'm having the same issue. I did the 11" ROH, after installation I bleed the brakes but no pressure, im so bumbed cause i really wanted to feel the results tonight. I'm gonna give it another try tomorrow with the fluid pot method and hopefully that will be it, I don't want to buy a master cylinder just yet. Wish me luck.