braass91

1991 Honda Civic Hatchback DX

While I am at traffic lights, the brake pedal slowly depresses towards the floor. What would cause the brake pedal to do this?

And, while stopping coming off the freeway off-ramp, the engine suddenly revs up and go back down to normal idle. Any ideas? Thanks for your help.

xxOldschoolxx

Im thinking it could be your master cylinder that went bad

stormdrop

the brake booster can cause that also if its not getting vacuum

olddohc

iTrader: (116)

first bleed the lines starting from the farthest away from the master to the closest(pass rear, driver rear, pass front, driver front). While you are doing that inspect each line and the tightness of the line where it connects to the hard line on the car. also check where it connects to the calipers. this is just to be complete in your inspection. its probably the booster but make sure it is a not something stupid like a brake line. Another indication is if you are going through fluid, sometimes it can leak from the booster/master. make sure you check the tightness of the prop valve too just to be thorough in your inspection.

Tyson

ur master cylinder has an internal leak, that needs replacing for sure. be sure to bench bleed, use a flarenut wrench, yadayada. tons of threads on this.

it could be leaking fluid into the booster and disrupting vacuum from the intake manifold.

replace the master cylinder first, as that for sure needs replacing. very typical in hot months



(dont listen to anyone above, just replace the master cylinder. 100% needs replacing.)

olddohc

iTrader: (116)

yeah this guy is the man of men. most interesting man in the world at that. "he once changed a master cylinder and didnt listen to what anyone else said above." i guess in this case dont ever listen to anyone else just PM tyson for all the right answers lol.

94eg!

iTrader: (2)

Tyson is 100% correct. It's a bad MC piston seal. I've dealt this multiple times and it always 1st appears when it gets hot out.

When it's acting up, you can have someone push the pedal to the floor & back and you can actually watch the fluid rise & fall in the reservoir.

olddohc

iTrader: (116)

And so is everyone else that said the same thing. Its a 22 year old car y not inspect the other stuff too just to make sure? Oldschool stormdrop and olddohc are 100% correct.

whited

Why has noone suggested the obvious line\hose leak?

94eg!

iTrader: (2)

TearSDMF89dx

who ever suggested the booster causing a low pedal is 100% wrong a failed booster or bad vacuum will always cause hard pedal ....a low pedal that comes back on the second pump is stuck caliper slides ....and a slowly fading pedal is either a worn out master cylinder ,air in the system or a fluid leak somewhere in the system

Tyson

really guys?

braass91

I'll check all three - master cylinder, air in system and for leak.

How about the sudden engine rev while braking just coming off the freeway off-ramp? It happened twice, both times as I am stepping on the brake, but I think, it happened when the car is stopped or close to being stopped. Thanks for the information.

eflink

If the brake fluid in the container does not diminish, check the 0-rubber inside master if there is a leak within.

sstupid

Everyone shut up before you get this guy killed. OP: listen to Tyson.

Zaxxx20

Might be faulty vaccum line. I ve had vaccum lines only fail after heating up. Only reason i say this is due to rpm jump. But more than likely bad seals.

4drEF

A vacuum line made your brake pedal sink to the floor?
?????

The only thing that makes the pedal sink is failure in the hydraulic system.
Master cylinder is the typical culprit as Tyson pointed out already.

94eg!

iTrader: (2)

^Yeah. If you vacuum line fails, it could tripple the amount of vacuum going to the booster. Oh wait.......that makes absolutely ZERO sense!!! lol

DCRB

in all due respect a sinking pedal is caused from a leak in the hydraulic system. brakes work off of pressure. if there's a leak some where, either the mc, a cracked and leaking soft line, or a leaking caliper / wheel cylinder, the pressurized fluid is going to go to the path of least resistance. hence the pedal dropping to the floor.