Brakes Sinking to the floor when car is started

[Carl]

Hey guy very new to this forum, just bought an amazing car a 2005 corolla sportivo. I have a few external mods and plan to do a few more. I have a problem which i need some help with. I have recently taken my car to a mechanic as there was a slight noise in my right hand brake turned out to be nothing, then for some strange reason the mechanic thought he should take it for one more drive. When he brought the car back he said "mate have you ever heard a loud knock when the abs comes on" at this point i was thinking how fast was this guy driving my car? Anyway got into my car drove home didnt use the brakes till i got in my drive way and well and behold i almost went straight through my brothers hilux. My brake sunk straight to the floor. Now i have taken it back to him and of course he has taken no responsibility for it, he has later diagnosed the issue saying it was a master cylinder, and he wants to charge me a small fortune to replace it. this is a car with 30000kays on the clock, i sourced the part myself and had it put on properly, guess what the problem is still there in its orginal form. Please someone tell me what this idiot has done to my car and whether i should presue some course of compensation, my car has now been off the road for 1 and a half weeks and i have no car over the long weekend either..... HELP SOME ONE. THANKS

[SuperDave]

Sounds like the vacuum assist isn't working properly. Never dealt with a master cylinder before so can't help you out with what to look for.

JP, you got any ideas?

[Carl]

Sounds like the vacuum assist isn't working properly. Never dealt with a master cylinder before so can't help you out with what to look for.

JP, you got any ideas?

Thanks for the input mate, yeah it seems a bit strange, seeing it has such low kays. And it isnt the master cylinder as i had it replaced and still having the same problems. the strange thing is, it pumps up and then after left for a second your foot ends up back on the floor, and the idle is a little eratic aswell.

[CHA54]

when the vac assist isn't working or connected you end up with a very heavy pedal which is difficult to press, it still works, you just have to stand on it almost with both feet.

When the pedal drops to the floor you have a problem somewhere in the hydraulic system, if it only started happening after ABS actuation I'd first be looking to completely bleed the ABS unit (not just your standard caliper bleed).

So the pedal just pushes to the floor with no resistance at the moment?

[Guni]

Doesn't the clutch and brake share the same cylinder?

If its a vacumn issue, you would feel it at the clutch first if not also.....

Maybe one or more of your brake hoses are leaking or ruptured?

[CHA54]

no they dont. They share the same fluid resivour, but have seperate cylinders.

[Carl]

no they dont. They share the same fluid resivour, but have seperate cylinders.

Yeah the crazy thing is it pumped up really hard when bleeding the brakes and when the car was off, but start it up and the foot is on the floor, you car slowly pump it up, but leave it with no pressure for a few seconds and its back on the floor again. the clutch seems fine, no different, but the mechanic did mention the abs coming on and the loud knock and then no brakes.

To all of you who replied thankyou, your all very insightful and appreciate it greatly.

[T.J]

sounds like a possible air lock in the abs unit itself this happens sometimes certain vehicles need a person using a diagnostic device to open the solenoids in the abs unit whilst the pedal is held down and the nipple for that brake line opened to properly flush any old fluid or air through the system i know you need to do this on land cruisers not sure bout corollas.

its possible the brakes may never have been used to the extent that the abs system engaged before so if it is an air lock the mechanic isnt responsible cause it was already there just in a part of the braking circuit that was never used.

as stupid as it sounds making your abs do some work occasionally even if its not neccessary isnt a bad thing cause it cycles the fluid through the system also if theres a problem like this itll present itself before you really need your brakes.

also abs can work at low speeds so the guy may not have been driving very fast at all