Ok, all i have done is search through pages and pages of threads on brakes...master cylinders and blah blah.

But heres the summary: Started off...brakes were fine for 20-30 mins, then at stops the pedal would slowly sink to the floor. Thought that it must be the master cylinder.

PT 2: After swapping in a 95 GSR master cylinder and brake booster...i still have the same problem.

WTF is going on? I have a 93 Civic EX sedan with ABS if that helps.

 

I needed to rebuild my prop valve after i blew my master cylinder putting on ss lines on a 195,000 mile old MS.

double checked no air in the system???

 

make sure your hoses arent leaking.
make sure you bench bleed the master cylinder before installation.
it sound that you have air in the system.

i repaced my master cylinder and it went out like in 3 months. get a new one

 

check your slave cylinder....they kind of go hand and hand... if you replace one, you should replace the other at the same time

 

read!!
brakes not clutch

 

It could be the booster, but it seems like a bad seal in the master cylinder. Replace it before you have no brakes. Happened to me in my 91 Prelude.

 

no definite answers?...

and i dont think its air in the system because its really stiff sometimes...but then others its soft and slowly goes down to the bottom

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by impound &raquo;</TD></TR><TR><TD CLASS="quote">check your slave cylinder....they kind of go hand and hand... if you replace one, you should replace the other at the same time</TD></TR></TABLE>

oops sorry i'm an idiot......look on the firewall inside and see if there is any fluid running down the pushrod....thats a sure sign of a master cylinder gone bad....

 

replace it with a NEW master cylinder, not a used one. not a rebuilt one, get a NEW one.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by brockscivic &raquo;</TD></TR><TR><TD CLASS="quote">no definite answers?...

and i dont think its air in the system because its really stiff sometimes...but then others its soft and slowly goes down to the bottom</TD></TR></TABLE>

If you're not visibly leaking fluid at any of the lines, valves or calipers (check the inside of the firewall as well as the outside), then you probably have a crack in the intake manifold near the brake booster nipple or in the line connecting the booster to the manifold. If there's a crack in the 90-degree fitting coming out of the intake, the pedal pressure will depend on the heat of the manifold and the fitting, as expansion and contraction of the metal(s) will effect the loss of vacuum in that region.

It's happening to my 93 Si right now, actually, and I'm probably going to have to replace my intake manifold to get it to stop.

 

I have a similar problem to yours brockscivic.
Mines: Pedal has lots of travel till signficant breaking force is being applied to the wheels. Is also compressable even at that point till it can't go any further (full stroke achieved). Last year tried bleeding breaks numerous times traditional method, this year I have vaccume pump, I will use it to bleed brakes sooner or later. No break failure though; They work fine, just pedal has extensive travel and is compresable. No fluid leaks suspected either.

If ur pedal is SINKING.. check your fluid level, see if it's going down or alteast changing significantly through a series of quick pushes of the pedal.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Archidictus &raquo;</TD></TR><TR><TD CLASS="quote">If you're not visibly leaking fluid at any of the lines, valves or calipers (check the inside of the firewall as well as the outside), then you probably have a crack in the intake manifold near the brake booster nipple or in the line connecting the booster to the manifold. If there's a crack in the 90-degree fitting coming out of the intake, the pedal pressure will depend on the heat of the manifold and the fitting, as expansion and contraction of the metal(s) will effect the loss of vacuum in that region.

It's happening to my 93 Si right now, actually, and I'm probably going to have to replace my intake manifold to get it to stop.</TD></TR></TABLE>

WRONG, i hate to argue w/ you, but in the event of loss of vacuum or low vacuum the brake pedal would get stiffer. try it, disconnect the vacuum line going to the booster and cap it off, take if for a drive. be careful though, you will have manual brakes.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by schardbody &raquo;</TD></TR><TR><TD CLASS="quote">WRONG, i hate to argue w/ you, but in the event of loss of vacuum or low vacuum the brake pedal would get stiffer. try it, disconnect the vacuum line going to the booster and cap it off, take if for a drive. be careful though, you will have manual brakes.</TD></TR></TABLE>

There's a big difference in manual brakes - which I've driven on extensively - and vacuum assist brakes with a leak in the lines. Cap the lines and you have manual brakes, but leave the line damaged and you have a constant flux of pedal pressure resulting in symptoms like the guy above is describing.

It's happening to my car currently, I've already taken steps to isolate and deal with the problem, and no amount of you having driven anything else is going to change the facts. A completely open to atmosphere or completely closed to atmosphere system is going to act one way and a system of variable vacuum is going to act another entirely.

 

if i had a leak at the described spot...wouldnt it sound like a vacuum leak and/or make the idle slightly off

I think im going tomorrow and swapping out the master cylinder again...since its a free option. And if that doesnt work ill just have to deal with it until i get the cash for a new one.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Archidictus &raquo;</TD></TR><TR><TD CLASS="quote">There's a big difference in manual brakes - which I've driven on extensively - and vacuum assist brakes with a leak in the lines. Cap the lines and you have manual brakes, but leave the line damaged and you have a constant flux of pedal pressure resulting in symptoms like the guy above is describing.

It's happening to my car currently, I've already taken steps to isolate and deal with the problem, and no amount of you having driven anything else is going to change the facts. A completely open to atmosphere or completely closed to atmosphere system is going to act one way and a system of variable vacuum is going to act another entirely.</TD></TR></TABLE>

yes but he said its slowly going to the floor, which means the fluid inside the master cylinder is slowly bypassing the seals. easy test is, sit at idle, parking brake on, hit brake pedal sharply, barely ease off about 1-2mm, then reapply pressure, doing so will make the pedal drop quicker, if thats what happens its a bad master cylinder.

i agree though, a constant change in vacuum will make the pedal rise and fall.

 

thanks bro...ill do that test first thing in the morning...and ill post up the results