Master Cylinder for Front Brake not Pumping?

The master cylinder may need to be bled after changing the piston.

Install the MC on the handlebars so the reservoir is as level as possible.

Fill the reservoir to the full mark.

Remove the brake hose and press your finger over the outlet on the MC (block it closed with your finger)

Pump the brake lever a few times and release pressure on the outlet.

Keep repeating the procedure.

You're doing the same procedure as bleeding the brakes down at the caliper, but you're now doing it directly at the MC

Motorcycle Front Brake Master Cylinder

How A Master Cylinder Works

Too much air in the line to be able to bleed it.

Sounds stupid,huh?

When master cylinder piston retracts,,,it pulls a vacuum on the system.

Because system is Closed and filled,,,the vacuum has no place to go except the Refill Port between the Reservoir & the M/Cyl Bore.

So Fluid is drawn from Reservoir,,,and refills line.

NORMALLY that's how it goes.

When there's excess air in line,,,the Suction from the retracting M/cyl Piston gets Spent on pulling on the Air Bubble,,,,rather than on the Reservoir.

It's similar to a drinking straw with a hole in the side.

With excess air in a brake line,,,the master cyl behaves the same as if the line is not connected at all,,,,and sucking into Thin Air on it's refill stroke.

Everyone knows Master Cylinders PUSH/Pump/Pressurize fluid.

But it's often overlooked that they also VACUUM/Pull/Suck Fluid.

The Suction Mode is Crucial to Refill,,,self-adjusting,,,self-bleeding Function

It's a Pump,,,which has Lost it's Prime.

More accurately it's a Pump being expected to draw from Too High a position.

Master Cyl only has a TINY vacuum capacity.

It can work fine against a TINY air Volume in the line.

But it simply does not have the capacity to draw against an Air Bubble which is larger than what the M/Cyl can displace.

It's the same principle which keeps the Windows in our Homes from being sucked into the room as we breath.

Our Lungs are Too small to pull an adequate vacuum on the room.

Solution is Simple

Fill your brake line to remove the air,,,then bleed using normal procedure.

There's more ways to do that than I care to describe.

Simplest (and most awkward,messiest way) is to Pour fluid down the top end of the hose.

A common plastic bag with a small hole torn in the bottom corner,,,,Hose inserted into Hole,,,bag twisted Tight around hose then "clamped" with Tape,,wire wrap,,string tied,,,etc,,,,

Makes a decent Funnel.

You can even Squeeze the bag to pressure the fluid and "force feed" it into the line.

Us Lazy Americans with more money than we know what to do with,,,,and Everything in the world redily available within easy Driving distnance,,

might choose a Vacuum Bleeder.

Connect it to Bleeder and pull a vacuum whil Monitoring Fluid Level in Reservoir.

Same thing can be done by MOUTH---a long ,clear hose pushed onto Open Bleeder ,,,Mouth can draw enough vacuum.

It's Slow,,,but it works.

A LONG hose,,,looping down below caliper then back up to a convenient height so you can SEE the fluid and when the bubble comes out.

And mostly because Brake Fluid tastes Nasty.

However you Do it,,,the Line Must be Filled,,First.

"You cannot bleed a brake system,,if it has too much air in it"

It's simply beyond the capacity of the master cylinder to Do So.

IF,,,your Wheel Caliper piston is in normal position,,,Out against pad/disk....and not pushed all the in during servicing.....

SOMETIMES(if Air Volume in Line is not Too excessive) You can Push the Caliper Piston IN which displaces Caliper Fluid Into & UP the Line....

Air and Fluid will Burp out of the M/Cyl Reservoir.

Works like a Charm ,,WHEN CONDITIONS are Right.

Be wary of Overflow from reservoir.

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have you tried to open the bleed nipple and pumping the lever, similar to "bleeding" the brakes?...what happens then is fluid ejected?............

also is the diaphragm in good condition, no pinholes/tears?

if you live in Phils, you may want to get your self a good manual, eg Haynes, and do the work yourself....the mechanics Ive seen there tend to use "trial and error" too often

PSS....if the above works you owe me a 6 pack of Negra beer!

They build bikes with front AND back brakes. Don't worry about it and use the other brakes.