Brake hydraulics problem
 

A few weeks ago my '85 300d started exhibiting a peculiar brake problem. On initially pushing the brake pedal it is quite firm, and the brakes engage fine. With steady, moderate pressure, though, the pedal slowly sinks to the floor. Pump the pedal, and it does the same thing again and again. Brake fluid level didn't drop in the reservoir, and so I came to the conclusion that maybe the pressure was bypassing between the dual cylinders. I got a new ATE cylinder from Peach, and even a couple cans of ATE brake fluid, and swapped out the master cylinder, and bled the system with a mity-vac.
All seemed to go well, with the only problem being that it didn't solve the problem.
Any ideas on where the problem really is?
Thanks
SEO

I suspect imperfect bleeding job. Its possible your new cylinder is bad but before changing it I would bleed the system again. I have never been able to successfully bleed using a mitivac. I do it the old way using my Mrs.

I made a pressure bleeder. Sometimes I've been able to release the pressure and have the excess syphon back to the bleeder leaving the master at the correct level.

Well if you have a new MC then you don't have the over-stroking issue that can happen with an old MC when bleeding the old school way.

I would invest in a Motive bleeder and pressure bleed it then see where you are.

the MC needs to be bled.

it's fairly simple, easiest with the power bleeder, but if you get a set of bleeder hoses from an auto parts store, it can be done in the car.

disconnect the brake lines, hook up the bleeder hoses, and loop them back into the reservoir.
pump the pedal until all air is out of the lines, then repeat with each output line.

All the more reason to avoid making any "investments" in equipment which is not needed to get the job done.

what investments?

Pretty much the easiest way to get the job done. Especially nice to not need help on that job.

This doesn't feel like any air problem I've ever had, where the brake pedal is soft, maybe to the floor, but if you pump the pedal a few times it will compress the air enough so that the pedal gets hard and stays hard as long as you've got pressure on it. Release the pressure, the pedal goes soft again.
In this case, the pedal is hard at first, then goes to the floor with steady pressure, and doesn't pump up if you keep working the pedal. This car doesn't have ABS brakes. Is there a some other gizmo that could be bypassing here?
As far as bleeding goes, would it work to run a tube all the way from a brake caliper (starting at the diagonally furthest away) back into the reservoir, and bleed the system by pumping the brakes until bubbles stop coming off. .

It's not a question of ease. It's a question of cost-effectiveness for the typical DIY'er who:

1. Can get the job done without a pressure bleeder.
2. Would have very little use for one on an ongoing basis.

I typically do 3-4 brake jobs a month for friends and neighbors using the two-man method. My wife and kids are all trained in the brake-pedal end of the procedure.

No.

The pressure must be built up and held and the bleeder opened until the pedal takes a full stroke or nearly so, then repeated until the fluid comes out without bubbles....starting with RR then LR then RF and LF. (on a left hand drive car).

I use a broom stick cut to length between brake pedal and steering wheel in place of one of the two man. :D

In my case, it is a question of ease. Cost effectiveness comes into play because I'm essentially working at $75/hr after tax by doing my own work. There will be a time where I won't buy tools for a project but I'm not there yet. I still buy green bananas also.

My wife hates to work on the car and doesn't mind me having the tools so I'm not tempted to ask her. I postponed making a bleeder but it really makes the process soooo easy.

It is not impossible for any new or rebuilt Mastercylinder to have a problem and not work right.

However, the Mastercylinder itself needs to be bled first (instructions and a Kit to do the bleeding usually come in the Box with the Mastercylinder) and then the system.

As far as makeing a Brake Power Bleeder. Except for the Cap on the Mastercylinder the rest of the Power Brake Bleeder could be used on other Vehicles.

I have owned Vehicles since 1968 and have only used a Power Brake Bleeder at one place where I worked. Meaning I have always managed to get the Brakes bled one way or another without the Pressure Bleeder.