Topic: Master Cylinder Logic - or not..?

[Lars]

OK, old topic, but nevertheless...

Honda CB 750 K4 long time no use, so the ordinary front brake restoration job performed. Master Cylinder disassembled and cleaned up, new restore kit installed, but I can not get any pressure when installed back on bike. Done this plenty of times before with no major problems, so what now..?

Took it all off and back to hte bench with an philosophic attitude - how does it really work? I have the shop manual for the CB 500/550 which describes it all, and since the master cylinder is the same on CB 350F/500/550/750, the should apply to the unit.

Question 1:
There is on hole in the fluid container which leads the fluid into the "channel or chamber" where it is compressed by the piston into the hose. In the reservoir, close to this hole, there is something which looks like a hole, but id does not lead into the same channel. Does this have any function, or can I ignore it?

Question 2:
Assumed there is brake fluid in the channel/chamber, the fluid will be between the primary cup and check valve, where the spring is. When the brake lever is pressed, the lever press the piston which moves the primary cup which blocks the passage from the reservoir and pressurizes the fluid within the channel of the master cylinder. This pressure is transmitted to the brake pad. Inspecting closely, the hole in the reservoir is leading into the channel/chamber on the lever side of the channel, before you come to the primary cup. How does the brake fluid then get into to the channel where it is supposed to be compressed?

[mystic_1]

Question 1:
There is on hole in the fluid container which leads the fluid into the "channel or chamber" where it is compressed by the piston into the hose. In the reservoir, close to this hole, there is something which looks like a hole, but id does not lead into the same channel. Does this have any function, or can I ignore it?

Both holes lead into the master cylinder bore, one is just very very small and is often clogged.

Question 2:
Assumed there is brake fluid in the channel/chamber, the fluid will be between the primary cup and check valve, where the spring is. When the brake lever is pressed, the lever press the piston which moves the primary cup which blocks the passage from the reservoir and pressurizes the fluid within the channel of the master cylinder. This pressure is transmitted to the brake pad. Inspecting closely, the hole in the reservoir is leading into the channel/chamber on the lever side of the channel, before you come to the primary cup. How does the brake fluid then get into to the channel where it is supposed to be compressed?

When the piston is fully retracted, one of the holes does in fact lead to the master cylinder bore in front of the primary cup.  In fact, when you operate the piston you should be able to look down through the hole see it sliding back and forth.

mystic_1

[Lars]

Hi Mystic_1,

You are right, the number two hole was very tiny and clogged. Took a hair from a steel brush and got it cleaned up, problem is solevd Thanks, Lars

[Bodi]

The piston is a cup, when going "backwards" it can collapse a bit and let fluid past.

[bryanj]

It is common for a rebuilt cylinder to not pump fluid initially

(1) put on good quality rubber gloves

(2) with cylinder on bars, reservoir full, no hose connected and lots of absorbant paper covering everything

(3) put a gloved finger over the outlet and pull in lever

(4) release and replace finger

(5) release lever


Repeat 3,4,5 till fluid squirts under finger (that's why the paper) whilst holding lever in connect flexible pipe

Open bleeder and bleed normally

IF YOU DON'T WEAR THE GLOVES DON'T WHINGE WHEN YOU GET DERMATITIS AND THE SKIN FALLS OFF YOUR HANDS