My brakes are being a bitch.

[RodFarva]

So my brakes aren't working properly. Through some searching on the almighty Google it appeared to be the BMC, so I replaced it. Now I'm only having half of the original issue.Initially if I even touched the brake pedal it would sink to the floor and not come back up unless pulled all the way back causing the brakes to lock up and stop the car. I literally just finished the install of my BMC and the same thing happens except for when the car is off. Now it will get firm when the engine is not running but if I start the engine and push on the pedal it will go to the floor and not come up until pulled. Please help me figure this out TR.

[KRUKED]

Did you bleed the system...pass. rear 1st, drvr rear 2nd, pass. front 3rd and last but not least... Or did you not hook the peddle back up to the push rod maybe?

[Empire]

i'd almost say it is a combo of air in the lines and a dead/dying caliper.
when you replaced the BMC did you replace the booster as well?
the booster could be shot too. I've never experienced the check valve failing, so i'm not sure what symptoms that would give you, but you might as well rule that out too.

[KRUKED]

I've been told that if one shakes the BMC and you hear A LOT of shit in there rattling around, it's nearly shot.

[sean1]

Did you bench bleed the bmc before you put it in? Get it from advance?

[RodFarva]

Did you bleed the system...pass. rear 1st, drvr rear 2nd, pass. front 3rd and last but not least... Or did you not hook the peddle back up to the push rod maybe?

I didn't bleed the calipers yet, after the install I hoped in and tested the pedal got the same BS jumped out slammed the door and gave the car the finger.

i'd almost say it is a combo of air in the lines and a dead/dying caliper. when you replaced the BMC did you replace the booster as well? the booster could be shot too. I've never experienced the check valve failing, so i'm not sure what symptoms that would give you, but you might as well rule that out too.

Nope just the BMC I don't think any of my calipers are dying. When I started looking for the source of this problem I thoroughly inspected all four corners. Also my reservoir was full when I started the BMC replacement. I think I'll hunt down a check valve and hope that's the issue the booster is mad expensive.

I've been told that if one shakes the BMC and you hear A LOT of shit in there rattling around, it's nearly shot.

I already returned the old BMC as a core so I can't try that lol.

Did you bench bleed the bmc before you put it in? Get it from advance?

Yep bench bled as per the supplied instructions. Advance is really my only option next to pepboys.I'm going to vacuum bleed the brakes tomorrow when it isn't raining. Any of you have a spare booster you could sell me if that's the problem? Hang on to it if you can I might have to buy it.

[sean1]

I didn't bleed the calipers yet, after the install I hoped in and tested the pedal got the same BS jumped out slammed the door and gave the car the finger. Nope just the BMC I don't think any of my calipers are dying.

so you installed the new master cylinder and didnt bleed the brakes after?

[KendallH]

so you installed the new master cylinder and didnt bleed the brakes after?

Yeah, this. If you don't bleed the brakes.... they don't work.

Also boosters never go bad.

[omgosh]

Also boosters never go bad.

Seriously?

[RodFarva]

I know I haven't finished the job because the calipers haven't been bled. But the issue I'm having difficulty understanding is how they work fine when there is no vacuum but when vacuum is supplied they malfunction. Something I forgot to mention earlier is if I rev the engine the pedal moves up and down with the engine speed I.E. revs up pedal rises a bit back to idle pedal goes down, higher rev up pedal goes even higher.

[Empire]

I didn't bleed the calipers yet, after the install I hoped in and tested the pedal got the same BS jumped out slammed the door and gave the car the finger.

don't try to diagnose something as wrong until the entire job is complete.
replacing the BMC and not bleeding the entire system is paramount to rebuilding and engine and starting it before putting oil in it and then trying to figure out why it won't maintain idle.
Get the system thoroughly bled and then we'll go from there.

Also boosters never go bad.

even more
you're kidding right?
a rubber seal, that is in an engine bay with a widely varying temperature range, that as different levels of vacuum applied to it never goes bad? Wow.
I wonder what I was doing wrong with the handful of boosters that I've replaced. I mean, they quiet obviously wouldn't hold any sort of vacuum, and just whined, and one of them wouldn't release the pressure as it built up until it eventually ruptured.
So yes, boosters do go bad.
perhaps no where near as often as other parts, and is typically not the issue, but they do in fact go bad.

I know I haven't finished the job because the calipers haven't been bled. But the issue I'm having difficulty understanding is how they work fine when there is no vacuum but when vacuum is supplied they malfunction. Something I forgot to mention earlier is if I rev the engine the pedal moves up and down with the engine speed I.E. revs up pedal rises a bit back to idle pedal goes down, higher rev up pedal goes even higher.

they aren't really working fine when there is no vacuum. You just think they are working because you don't have the same strength as you do when you get the vacuum assist from the booster. When you are using just your foot, no assist, then you can only apply so much force. The pedal will feel firm, and the calipers will engage, some. Which will make you think that it is work.
Then you apply the vacuum assist and it multiplies that force several times. This is what makes the pedal more soft to push on as compared to when there is no vacuum. Now what is happening is that you are compressing the pockets of air in the lines, which is letting the pedal go basically to the floor and not supplying much if any stopping power. Ironically enough, it is the same amount of stopping power that you thought was there when you pushed with just your foot.

now, since you say the pedal moves in relation to engine speed, that would indicate to me that either A) you need to adjust the rod that connects the pedal to the BMC. if it is a bit too short, the brakes will be partially engaged and when the booster fluctuates the amount of vacuum assist it can supply, the pedal will move. and/or B) it would also make me question if the booster was tweaking out and staying engaged itself. Holding too much vacuum.
Did the car sit for a long time?
One the few that I had to replace, the car has been sitting for 6 months to a year.
I got lucky and had a parts car to pull what I needed off of so I've never had to buy a brand new booster.

Also, something to consider, the BMC's you get from advance are re-manufactured units. meaning, they are metric pieces of metal that are torn apart and filled with SAE seals, so they can very well be bad when you get them.

[KendallH]

even more
you're kidding right?
a rubber seal, that is in an engine bay with a widely varying temperature range, that as different levels of vacuum applied to it never goes bad? Wow.
I wonder what I was doing wrong with the handful of boosters that I've replaced. I mean, they quiet obviously wouldn't hold any sort of vacuum, and just whined, and one of them wouldn't release the pressure as it built up until it eventually ruptured.
So yes, boosters do go bad.
perhaps no where near as often as other parts, and is typically not the issue, but they do in fact go bad.

My mistake. Doing brake overhauls on two of my cars I had never read about a booster ever going bad but I suppose it would be possible, however rare.

[chome]

Even though I really doubt your booster went bad, I have a spare S14 auto one at my house you can have if infact it did. Now that I sold my car I need to get rid of all this junk anyways.

[RodFarva]

I did get ahead of myself in posting this thread. I don't know what I was thinking. Chome PM me about pricing on that booster if it'll work for my S13 w/ MT

[obituary]

pass. rear 1st, drvr rear 2nd, pass. front 3rd and last but not least? iv alwas done it the other way around ....neerest to Master cyl 1st..ect.