How the brake MC is supposed to work?

[Philippe-J.]

Hi.

In testing by IRS brake circuit, I tried and used the master cylinder as a pump to put pressure into the circuit. I fed the MC with brake fluid form one reservoir, hung above the MC. However the MC didn't seem to work as I intended.
As a matter of fact, brake fluid was hardly pushed out the MC output port. Sometimes it did when I pressed the pedal, then stopped. Moreover when I got some fluid at the end of the flexible hose that goes to the IRS (not yet connected to it), I noticed that the MC piston was reluctant to come back to the high position. Then air was pulled into the hose, and the pedal could come up freely.
I am a bit confused as I did not find a clear explanation of how the MC is supposed to pump fluid. Can anyone tell? And confirm that the behaviour of my MC is not right? Of course if anyone understands the reason for this, I"ll be happy to read him

Thank you in advance.

[Gfhug]

Philippe, is your system the earlier one as shown on page L.X.s.2 of the service manual or the later one as on the next page L.X.s.3?

Is the fluid going via the servo or are you just linking the pipe directly from the mc to the IRS?
Is your master cylinder new or refurbished?

The answers could help the more knowledgeable to help you further.

Geoff

PS, searching for “master” then within the results for “brake” will show you lots of discussions about the brake master cylinder. I’m sure this will have been talked about before but you will have to patiently go through the results.

[Philippe-J.]

Geoff,

my system is original to my feb. 67 OTS, I guess it's a "latter" type, it looks like this:

Your questions, my answers:
Is the fluid going via the servo or are you just linking the pipe directly from the mc to the IRS? --> direct linking from the MC output to the T connector of the IRS



Is your master cylinder new or refurbished? --> refurbished with the SNG kit. The cylinder interior looks ok.

I already searched for the specific matter of how BF is supposed to flow through the MC, and how the MC should load the downwards circuit with pressure, but with little success. There are 2 holes in the input port, on each side of the main cup, I don't understand why.
My main question is: is the piston supposed to put pressure at each mouvement of the pedal, thus expelling BF out, until no BF remains in the reservoir? It is clearly not the case with my MC

[mgcjag]

Hi Philippe do you have the service manual..there are very good detailed drawings of the brake system showing how the fluid flows....Steve

[Gfhug]

Philippe, I would expect that to work by pressing the brake pedal the brake fluid should be expelled and eventually the reservoir emptied.
One thing I’d ask you to do is check out the overhaul of the master cylinder. Here’s a topic that can help explain how the small piston can get stuck even with red rubber grease on it.
Have a read and see if that could be your problem.

viewtopic.php?f=3&t=19987&p=162389&hili ... ed#p162389

Geoff

[Philippe-J.]

Thank you for your help and suggestions.
Steve: the drawings in the service manual revealed that there exists a second (very small) hole in the input port of the MC. I guess that it should feed the downwards part of the MC. Indeed, after disassembling, I found it blocked with a rest of mud. I cleaned it.
Geoff: your confirmation about the regular behavior of the MC pushed me to reconsider my initial refurbishment. To my great shame I found that I reversed the main cup. It has a W shape, and the large base was towards the top of the cylinder . So it retracted when pressing the pedal, instead of expanding against the ID of the cylinder. No pressure-proofing could be obtained that way for sure!
Those two discrepancies being corrected, my MC showed a correct pumping action, and the bleeding of my IRS circuit ran as by the book
I am quite relieved and grateful for your help