NO front brakes 300CD

[sun tortise]

Just bought an 83 300CD with NO front brakes. Previous owner replaced the calipers, hoses, and i think the booster and M. cylinder. Not sure about the master cylinder, the installed one looks newish, and there is another in a box in the trunk that also looks newish. The rear brakes work, and will barely stop the car if pushed to the floor. The front brakes, which are connected to the rear part of the cylinder, receive ZERO fluid. I removed one of the lines from the front brake connection at the rear of the master cylinder and not a drop of fluid flowed when pumped. The reservoir is very full, above the max mark. Is it safe to assume that the master cylinder is bad?


Advice appreciated, it seems everything is counter intuitive lately!

[tyl604]

Maybe they did not bench bleed the master cylinder when it was replaced.

Or there is a blockage in your front brake lines. I would open the bleeder on a front brake and see if I could force any brake fluid through. Actually might be easier to do it with an air hose and try to blast some air through.

[sun tortise]

I removed one of the lines at the master cylinder, 'cuz i saw another post that the equivalent of a bench bleed could be done with the M.Cyl installed, running a tube from the brake line connection on the M. cyl to the reservoir. Shouldn't some fluid have come out of that brake line connection when pumped even w/o the tube running back to the reservoir?

[tyl604]

Yes. But I encourage you to do a bench bleed and reinstall the MC. Then try bleeding all four brakes. Also make sure both halves of the MC are full; often the back one gets forgotten.

[Frank Reiner]

st:

Look very closely at the reservoir from the inboard side (the right side). There is a divider in the reservoir that separates it into front and rear sections. It is possible for the front section to be full, and the rear section empty. The fluid level in the rear setion can be seen more clearly from the inboard side.

[sun tortise]

The rear chamber is chock full. If i added another teaspoon of fluid, the reservoir would overflow. Shouldn't at least a little fluid come out when pumped? I am not sure how to do a "benchbleed". Did one once decades ago. Couldn't it be done with the brake lines unscrewed but pumping the cylinder with the pedal?



Thanks

[NZScott]

Place a rag under to avoid leaks onto your paint, and loosen the brake line connections at the cylinder and see what happens when pushing the pedal. That's all a bench bleed is really. I have some old cut brake lines which loop back into the reservoir for doing it.

[barry12345]

If the brake hoses look new and no fluid is coming out of the bleeders. Almost has to be the master cylinder. The port is blocked somehow. Bleed the master properly and if it will not bleed properly change it.


Normally you would see some gravity flow out of those front bleeders. That you do not bothers me a little. You might try taking a front bleeder right out. sitting down and watching for ten to fifteen minutes.

[vstech]

No liquid will come out if it needs a bench bleed...

However, if the old plastic reservoir was used, and it has something blocking the hole, liquid wouldn't make it to the MC.
Get bench bleeding lines and use them to get air out of the MC.

[Diesel911]

If passage B in the diagram is plugged that portion of the master cylinder will not fill.

If you are losing fluid it is possible the fluid is going out the rear of the master cylinder and into the brake booster.

If spring number 17 broke the rear piston would not return and would not fill up with fluid.

[rocky raccoon]

Pressure bleeding will overcome the problem. It is an easy one-person job. Just need to buy an inexpensive tool (abt $55) that you will use many times in the future on all of your vehicles.

Motive makes a good one.

[sun tortise]

Bought a brake line i will cut in half and attach to the front brake outlets on the M.C. and loop back to the reservoir. Hopefully i can bleed it sufficiently via the pedal. If that doesn't work, i guess i will try the other cylinder. Probably have to wait till Monday as it may rain Sunday.



Thanks for all the advice!

[Junkman]

I went through a few remanned MC from the local parts store and ended up buying a new part from this site. It has worked flawlessly for several years. These master cylinders for whatever reason need to be bench bled. Other cars I've had bench bleeding didn't matter.

There are frequently nipples that match the threads included with the MC. The manufacturer knows that this process can't be skipped and supplies the parts to avoid returns. I keep them for when needed down the road and have never had a reason to spend money buying a kit. Master cylinders have not been a frequent replacement item perhaps because stopping the car is so important that manufacturers build brake systems to last a long time without maintenance.

[rocky raccoon]

Pressure bleeding eliminates the need for bench bleeding.

[Junkman]

Originally Posted by rocky raccoon

Pressure bleeding eliminates the need for bench bleeding.

Not necessarily. My 85SD for whatever reason was a pita to bleed. I ended up pulling the MC, bench bleeding to get the MC working then pressure bleeding to clear air and change the fluid in the lines. Pressure bleeding alone did not work. I have no explanation why but ran almost a quart of new fluid through it before pulling the MC during the original pressure bleeding.