Brake pedal goes to the floor,and master cylinder was just replaced

[mrmarkuSS]

A week and a half ago,I had to stop short because someone else did.Five minutes later my brake pedal went to the floor,and the car barely stopped.Checking the fluid reservoir,the front half was bone dry. I added fluid,but it wasn't any help. So I took the car in,I was told it was the master cylinder,so I supplied one from a junker,and I got it back Wednesday,and it was much better. Today,while driving while stopping for a light,I hear a metallic thunk come from the left front of the car,and a few minutes later,the brake pedal goes to the floor,again.I park the car and check the reservoir,some fluid is missing,so I add some.Not much help.But unlike the first time,the brakes work with some pumping,and it pulls to the right if I try to press hard on them. So,what could it be? A bad master cylinder,again? Or something different?
Side note: when the mechanic had it,he said he didn't see any leaks.So where did all that fluid disappear to? I did see some fluid on the frame just below the point where the lines attach to on their way to the brake fluid cooler.But pumping the pedal didn't reveal any leak,no bubbling or anything.

[91ss]

Mine was the brake line to the back. Rotted just about at the start of the front door. Fluid laid in the frame/ dripped while driving. Wasn't until I refilled in the driveway and pumped hard till I was it start dripping. Maybe the same?

Same situation. Had to hard brake and it broke loose.

[fuzzcar]

Bleed the hell out of it!

[Hamilton Chevrolet]

Brake fluid does not evaporate quickly like water does so you surely have a leak somewhere if you are loosing fluid.

Mike--Hamilton Chevy

[mrmarkuSS]

Thanks for the input guys.Hopefully he will find what the problem is and get it fixed by Tuesday.

[Navy Lifer]

Originally Posted by mrmarkuSS

Checking the fluid reservoir,the front half was bone dry.

.....pedal goes to the floor,again.I park the car and check the reservoir,some fluid is missing,so I add some.Not much help.But unlike the first time,the brakes work with some pumping,and it pulls to the right if I try to press hard on them.

where the lines attach to on their way to the brake fluid cooler.

Low fluid in forward part of reservoir means fluid loss is occurring in rear brake system.

Second occurrence--was same section of reservoir (forward half) just low, or completely empty?

I suspect the "thunk" you heard was the combination valve shuttling to prevent fluid flow to the rear--if the problem in the rear section of brake plumbing prevents pressure buildup, the combination valve shifts to block that flow and ensure at least the other end of the car still has brakes and can stop the car. It will be necessary to check many more things to diagnose why car pulled to left under braking, but I suspect the car was being stopped with only thefront brakes.

Was the red brake system warning light (not the ABS light) ever on when any of the low-pedal conditions occurred?

Brake fluid cooler? Tell us more about this device, please!

The fact that you installed a used MC does leave open a possibility that it has a problem--or the system was not bled properly in the process, so you want to find out from the shop whether the replacement MC was "bench bled" before being installed on the car.

Take a look under the dash where the MC mounts to the firewall to see if there is any evidence of fluid running down the firewall on the interior side of the car. Also, check the power brake booster and vacuum line/filter to see if there is any evidence of brake fluid getting pulled into the engine via the booster. If the fluid loss is only from the forward section of the reservoir, it is most likely that the leak is somewhere rear of the ABS control, but exactly where, and considering that it has not been located yet, remains for you to determine.

[mrmarkuSS]

Originally Posted by Navy Lifer

Low fluid in forward part of reservoir means fluid loss is occurring in rear brake system.

Second occurrence--was same section of reservoir (forward half) just low, or completely empty?

I suspect the "thunk" you heard was the combination valve shuttling to prevent fluid flow to the rear--if the problem in the rear section of brake plumbing prevents pressure buildup, the combination valve shifts to block that flow and ensure at least the other end of the car still has brakes and can stop the car. It will be necessary to check many more things to diagnose why car pulled to left under braking, but I suspect the car was being stopped with only thefront brakes.

Was the red brake system warning light (not the ABS light) ever on when any of the low-pedal conditions occurred?

Brake fluid cooler? Tell us more about this device, please!

The fact that you installed a used MC does leave open a possibility that it has a problem--or the system was not bled properly in the process, so you want to find out from the shop whether the replacement MC was "bench bled" before being installed on the car.

Take a look under the dash where the MC mounts to the firewall to see if there is any evidence of fluid running down the firewall on the interior side of the car. Also, check the power brake booster and vacuum line/filter to see if there is any evidence of brake fluid getting pulled into the engine via the booster. If the fluid loss is only from the forward section of the reservoir, it is most likely that the leak is somewhere rear of the ABS control, but exactly where, and considering that it has not been located yet, remains for you to determine.

The rear brakes.At least that's a start.
The second occurence both front and rear parts had a small amount of fluid missing,about halfway in the reservoir.
Yes,the red brake light came on.
Brake fluid cooler=equipped on 9C1 (which is what I have),cools the brake fluid via a heat sink.
So far,only evidence of a leak is where the lines attach to a clamp on the frame-the set that is from the master cylinder,goes into a coil loop then to the frame and runs to the front to the brake fluid cooler-it was wet there.

[NADACOP]

I have nothing to add and hope you solve this problem soon, but would it be possible to get a picture of the brake cooler?

Thanks,
Jim

[mrmarkuSS]

nadacop,when I get the car back,I'll get a few pics and send them your way...

Mechanic told me today that the master cylinder I supplied went toast. I'm still a bit skeptical that it did,junkyard parts usually last a good amount of time for me,so I think something's causing it.But who knows,maybe it was on its way out to begin with.

[Ken Hennessy]

Brake lines will rust under the clamps at the frame. That could be the cause of the wetness.
Ken