I was doing a routine brake bleed and had a mishap when doing the last wheel. My spanner slipped cracking open the nipple, lost the hose. Then I think my assistant raised the pedal and a buch of air was introduced into the system.

Back to square one I tried to bleed the master cylinder and I cannot seem to get the air out. Bubbles just keep appearing after a dozen fills of the reservoir. Been at it for hours

So much air has come out that I'm starting to suspect that air is getting into the system somehow. After that much bleeding should I start suspecting a deeper problem?

I eventually seemed to get all the air out but it took much longer than expected.

If the 120Y master cylinder is the same as the 1200 there's a purge nipple on the master cylinder you have to do first. Different to every other car I've worked on. That may have been the issue. One of my least favourite jobs.

A hint for people bleeding brakes, Often when a master cylinder is older a common issue here with older cars a wear lip will be present in the master cylinder.

Naturally when you pump the brake pedal full stroke when bleeding the system the seals can be damaged going over this wear point,As a mechanic it makes it hard explaining to a customer that they need a new or rebuilt master cylinder because in bleeding the brakes that previously worked now they do not work.

The solution to this problem is to gravity bleed the brakes when doing fluid changes that should be done on at least a annual basis due to the fact brake fluid is hygroscopic namely it absorbs water that then causes corrosion in master cylinders wheel cylinders and calipers.

Gravity bleeding simply means open the bleed nipples and brake fluid will then run out by itself,Often on a two post hoist with all wheels off you bleed all brakes at the same time.

Now I hear you say but what if the fluid runs out in the master cylinder then you have to do a conventional pump the pedal bleed the very thing you are trying to avoid and the way around that is either keep the fluid topped up manually or surprisingly take a clean bottle a 600 ml Coke bottle is my favorite full of brake fluid turn it upside down in the master cylinder reservoir and it will keep the fluid level topped up but will not overflow only maintaining the fluid level in the master cylinder.

I won't go into the physics of why that happens but trust me it will not overflow, you can test the Theory with a bottle and some sort of container with water first if you wish but it does work and most importantly it will save you from a master cylinder rebuild or replacement requirement at times

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