Brake Pedal Pressure Drops?

[Greensand73]

Hello
On my 73 Etype i had the brake master cylinder replaced. Yesterday when i was driving the car, and had my foot on the brake...after the car stops, the brake pedal suddenly goes down about half way, Any idea what can cause this?
Sandy

[42south]

Hi
The normal cause is some air still left in the system.
Get your mechanic to rebleed the brakes.
Cheers
Mark

[Series1 Stu]

Or the brake fluid is moving past the piston in the master cylinder due to weakened seals. Were you holding the car on the brakes at the time?

Regards

[Greensand73]

Yes i was holding the brakes. One more comment on this...the brakes do not go down gently, when i have my foot on the brakes after the car stops, it suddenly jerks down about an inch....i did have a master cylinder put in about a year ago, and there are no fluid leaks
Sandy

[abowie]

I'm not sure what is causing your problem, although I'll have a few guesses below. I would not drive the car until the problem has been positively identified and solved.

What you describe is that the car stops with the brakes, that while this is happening the brakes feel normal (you don't say otherwise) THEN with pressure on the pedal it suddenly drops an inch.

For this to happen, a volume of brake fluid of up to 10ml is suddenly being displaced somewhere. This is quite a lot of volume. Causes could include something stuck that is suddenly releasing (a piston for example), a brake fluid leak, or water contamination vaporising in the lines and becoming compressible.

If something is sticking, it's moving a LOT to allow that much fluid to be displaced, and should be pretty obvious. Look to see if any of your pads are broken or misaligned and check that all the wheels actually lock when you apply the brakes. Did the car stop straight; did the brakes make any odd noises?

If it is fluid leaking you will notice your reservoir bottles levels dropping. If you really are losing up to 10 ml that should be evident pretty quickly. Fluid can leak internally in the booster. This is common and the booster can contain 100's of ml of fluid without it being evident. Similarly fluid can go into the Reservac tank if your car has one. Fluid leaking out the master cylinder will go down into the foot well, but a lot can be soaked up by the trim before you start to notice it. Do you have any funny aftermarket bleed nipples fitted that have valves in them, and might leak under pressure? Bottom line is leaks may not be obvious.

Water contamination is more common with non silicone brake fluid and in cars that have sat for a length of time. Mineral brake fluid is hygroscopic. In your daily drive, the small amounts of water absorbed overnight are driven out by heat, but if you haven't driven the car for months it stays there. A small amount of water can turn to steam, and while water is incompressible, steam isn't. So you lose your brakes after they get hot.

I suppose it is possible that there is something wrong with your vacuum system that is suddenly applying vacuum late in the braking, but I don't think it would feel like you described, and your pedal would be quite hard to depress prior to this, which you don't mention.

Air in the lines usually gives you a spongy pedal immediately you put your foot on the brake, not a firm pedal that then gives way.

I would ascertain if you are losing fluid from the reservoirs. I would visually inspect all the components and ensure that the pistons and pads move properly. I would thoroughly bleed your whole brake system, aiming to completely replace the entire volume of brake fluid. I would check the Reservac tank and the function of the vacuum system.

[Greensand73]

Thank you Andrew. The way you describe the issue is exactly how it happens. you are right on not driving it. Will update as i learn more
Sandy