dimmer

I was at a complete stop when the brake pedal suddenly and slowly drop to the floor like there has a leak in the hydraulic lines.
I lifted my foot, pushed down again and it was hard as a rock.
Check the brake fluid level and it is full.
It's happened 3 times now and only at a stop.
I was think maybe something up with the ABS?
When it happens, I don't feel or hear any pulsing like when ABS is activating under slippery conditions.
Is there a check valve that might be starting to go? Any ideas?

Thanks in advance!

GlenL

Bad master cylinder. Replace it. Yes, it's "that much."

davek9

How's your e brake working ; )

dr bob

What Year Is Your Car?

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The answer to your symptom is almost undoubtedly Master Cylinder, with a piston rubber cup suffering as it passes by the bypass/fill port from the reservoir.

On some years, the piston is two pieces with a circular-reed check valve between sections. The tiniest amount of rust/crud/contamination can prop the reed valve "washer" off the piston boss, allowing fluid to move from front to rear section. If both sections leak at the same time, the pedal goes to the floor as you describe:

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Recommendation is ALWAYS to replace the master cylinder and do a full bench-bleed on the new MC prior to mounting it, plus a full fluid flush on the rest of the system.

Alternative is to follow exactly the same procedure with the existing MC. Full flush of both the MC and the rest of the system. Bench-bleed the master cylinder before re-installation. Full system flush after that just to be sure.

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There's no substitute for no less frequently than annual brake system flush and fluid replacement. Keeps moisture levels low, keeps corrosion down, and keeps rust and crud levels manageable. Calipers and the master cylinder last forever if they aren't subject to the dirt/rust/crud erosion. Worthwhile to have this on your annual PM list. For the cost of a couple liters of semi-premium fluid (<$25), this is really cheap insurance.

Dave928S

I'll second that ... and add that it's always been my view that replacing all other fluids at frequent intervals is also good practice, and cheap insurance.

dimmer

Thanks as always guys.

My car is an 1988. I may try flushing the MC first as I suspect that it may be some crud propping up the check valve as Dr.Bob indicated being possible. I think it's this as it has only happened at a full stop when the pedal pressure is lowest (ie. less force pushing the check valve closed). Under road braking conditions, the pedal feels hard as a rock.

Mrmerlin

replace the master cylinder with a new ATE part.
Use DC111 on the reservoir grommets to the MC bore
add in some SS flex lines;,
and 1 liter of ATE gold fluid,
you should be good to go

WallyP

One of the more common causes for this is called "bypassing". After long use, the lips of the rubber cups in the master cylinder both wear and harden. Eventually, the cups get to the point where the lips do not flare out to a leak-free seal on the cylinder walls unless there is sufficient pressure in the system.

Unmistakable symptoms are: The brakes work fine on a normal stop with firm pedal pressure; The pedal will slowly sink to the floor with light pedal pressure; The pedal will firm up with increased pedal pressure; There is no external leakage of fluid.

Only cure is to rebuild or replace the master cylinder. Rebuilding a master cylinder is very rarely worthwhile...

StratfordShark

Please can I jump in on this, given I have found a NOS MC for my parts stash? My brakes feel wonderful after caliper refurb, but original MC will need replacement at some stage.

Bench bleeding looks straightforward, but is it possible to install reservoir outside car and bench bleed with lines going back into the reservoir, and then install complete reservoir/MC assembly back into car?

dr bob

Bench-bleeding is exactly as you describe, with the requirement that the MC is secured in a position that leaves the exit ports at the high points of the section it's connected to. Purpose is to make sure there are no hidden pockets of air, and to make sure there's adequate fluid velocity to get any entrained bubbles out. Ends of the bleed lines need to go below liquid surface so they don't add to the problem. I leave all the lines connected and routed until the MC is in the car, so there's no fluid dribbling out or air getting in before the steel lines get hooked up.

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ATE used to (and may still...) offer a kit of a bunch of plastic "flared end" fittings and clear hoses, along with a little clip that secures the ends of the hoses at the reservoir cap with the ends down into the liquid. I keep mine in a handy ZipLok bag in the brake-tools drawer.

StratfordShark

So if the reservoir is attached, how do you get the bleed lines to sit below fluid surface? Doesn't our reservoir have a fill neck shaped so that's it's hard to push a line down there, even with the filter gauze removed?

Mrmerlin

You can try the above suggestions or do it this way.

put the master in dry,
use DC111 on the grommets,
fit the metal lines,snug the B nuts
then fit the reservoir,
fill with ATE fluid,
put some rags under the MC.

Use gravity bleed or a pressure source connected to the reservoir.
Tap on the MC with a screwdriver handle to move any air bubbles
open the B nut closest to the booster,
wait till the bubbles stop then close it,
do the next nut and so on till the tip.
Then bleed the air from the MC tip if fitted.

Have a helper then press the pedal and start over on the B nut bleed sequence.

NOTE No pressure source is used during pedal pressing

then bleed the wheels
LF RF LR RR

StratfordShark

Thanks Stan. Mine is a later car (S4) so the MC has a bleeder valve that I use during regular system bleed. Is there any role for it here, or can you only bleed the MC by opening line nuts and looking for bubbles in fluid leaking from MC? Would be less messy if that bleed valve could be used.

Mrmerlin

cracking the B nuts fills the lines and lets air from the MC,
Bleed the tip last
please follow the instructions

StratfordShark

Got it, thanks a lot.

I misunderstood reference to "tip"!

Will follow this precisely.