Diagnosing stuffed Brake booster or Mst cyl

[mr_mike]

How do brakes react if the master cylinder or brake booster is stuffed?
At the moment my brake pedal has no feel to it at all when sitting at idle i can press with a lil force the pedal all the way to ground and when driving if i press the brake pedal, lift off then press again pedal goes to the floor.
Its an intermitant problem and sometimes pedal will feel firm while sitting at lights then suddenly (no change in engine rpm) the pedal will just go soft and again i can press to the floor.
The brakes only just pull the car up but i really have to stomp on them and press pedal all the way in.
At first i thought was possibly a air pocket in brake lines so today i bled the lines completly with new fluid. Started at wheel furthest from resevoir and worked my way inwards. This made no diffrence and pedal feel was exact same on way home.
All pads have heaps of meat on them so rules that out.

Have also inspected all lines and can c no leak and the resevoir level has no changed. The only thing i can think of is the brake booster or master cyl.

Does any1 have any suggestions b4 i go pullin shit apart tommorrow?

chees mike

[DISTRBD]

Clamp off rubber lines then see how the pedal feels , if it goes to floor its m/cyl ..

If not remove clamps to find which corner is causing the problem ..

[dattoman]

Hard pedal points to booster
Low pedal points to hydraulics

[joshg123]

before u shift and move stuff.

Get an assistant to press the pedal down, and check for caliper movement or flex. Ive had cars come in complaining of spongey brakes and bled to no avail. Yes a master will do this but it pays to check.

The pressure in the lines just shifts the caliper around on its mounting points rather than press on the pad. Ive seen it do this most often when you replace old pads after doing a wheel bearing or something when your taking pads out and putting them back on the same face rotor. The pad can sit up and the caliper will just flex all over the place and its shit.

If you can see a caliper flexing, clamp that line and press pedal again. If pedal goes hard. Rebuild that caliper

[mr_mike]

if it was caliper flexing wouldnt it b doin it all the time? and doesnt explain how the pedal would b feelin fine then suddenly drop to floor when sitting at the lights.
My theory is that a seal in the mastercylinder has shat itself, at low break pressure its holding but once pressure is increased it leaking.

[2jzlux]

sounds like your master cylinder is allowing brake fluid to bypass the seals
if the booster is stuffed the pedal will be really hard to push like when your motor isnt running
but it definately sounds like master cylinder

[RICEY]

my suggestion is stop driving it hahaha

[mr_mike]

thanks for that pearl of wisdom Ricey, way ahead of u car hasnt moved since friday avo and i havent had a chance to pull shit apart yet.

[LarryDavid]

don't rule out the brake booster..

i had the exact same symptoms in my car. although the usual sign of a faulty booster is a very hard pedal, in my case the pedal was falling to what felt like the floor before the brakes would work.

I believed it was an internal vacuum leak in the brake booster.. replaced the booster with a recond' one and it fixed the problem.

i could hear a hissing noise when i stepped on the brake pedal which is what led me to believe it was a vacuum leak somewhere. listen carefully under your dash or in your engine bay when you brake for a hissing noise. check the hose from your intake manifold to the booster for leaks. take the check valve off the booster and check it is working properly (only allows air to be sucked out of the booster). if these all appear to be fine it could still be a leak inside the booster.

[mr_mike]

I pulled off my brake master cylinder this mornin and i can c its been leakin from underneath a tiny bit.
Also the inside of the unit itself was full of gunk and appered to b all clogged up and weeping from behind the seal