Big trouble. Because of very questionable brakes, I have rebuilt my Brake Master Cylinder with all new seals, but the cylinder will not push brake fluid out to the wheels. It acts as though it's by-passing fluid internally (the pedal sinks to the floor with no resistance at all). I have installed the repair kit according to the printed maintenance manual for this car. All the parts and their locations within the cylinder are correct. The rebuilt cylinder was pre-bled before installing on the vehicle. Any thoughts how I might rectify this?

 

Did you hone the interior of the cylinder housing and make sure the lip seals are installed in the proper orientation?

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Thanks for the response. Yes I have confirmed that all internal parts are oriented correctly. I did not hone-out the cylinder because the sleeve is in pristine condition, with the plunger fitting very well. No hint of corrosion.

 

I would then inspect the wheel cylinders for leakage. It sounds like a case of a significant air pocket in the lines. You may have to "reverse bleed" the lines from the wheel cylinders like is sometimes necessary on a hydraulic clutch slave cylinder replacement.

 

Can you explain this further, please. My experience of all hydraulic braking systems is that bleeding (of air) can only be undertaken with the full assembled system, each slave cylinder in the drums being bled in turn until all the air is out.

 

Your whole system needs bleeding.

Even though you "pre bled " the master cylinder that is not enough as the system has been open and with 100% certainty will have some air in it.

Make sure the brake reservoir is full of fluid and pump the pedal repeatedly and rapidly until you get some slight resistance.
Get someone to hold the pedal down and crack a rear wheel cylinder bleed valve to let fluid and air out..

You may have to repeat this a number of times. You may also need to bleed all of the other wheel cylinders. Keep in mind the need to keep topping up the brake fluid reservoir.

 

Absolutely agree that a pre-bled master cylinder is great, but once you crack a line, the whole system needs to be bled -- from reservoir to *every* wheel. Also, while I'm not familiar with the MarkV brake system, my Mark IX hydraulics have a master cylinder *and* a secondary slave near the differential where the rear line splits for the two rear wheels. That sucker could have air in it as well.

 

I am not familiar with the MK-5 drum brake system but on my wife’s 59 Cadillac I can run into this same condition if the brake shoes aren’t adjusted correctly. If all the shoes have too much travel the master won’t get hard. So I would check the shoe adjustment on each wheel. Also are any of the wheel cylinders leaking?

cheers
Bob_S

 

The brake system in a MK5 is absolute simplicity and nothing like a Mk9.
I agree with Bob that if the brake shoes are way out of adjustment, then you would have difficulty in getting pressure.
But indications are that is not the case as it was working prior to the master cylinder fix.
Unless the master cylinder overhaul was a stuff up then the system should only need proper bleeding