Thread: Brake Master cylinder Piston Rod adjustment - a cautionary tale

This applies pretty much to all hydraulic brake (and clutch) systems, I think. If it says you need a little bit of free play, you really do need it!

Originally Posted by jedwards

Gentlemen,
I am back onto the brakes of my ‘53 S1 80” and had thought I was onto the end game. However, I suddenly found I had no brakes, and it is an easy thing to do quite unintentionally.

So better to be aware of the potential for getting things very wrong.

Last week I installed the newly skimmed drums and oversized brake shoes from Perth Brakes in Welshpool. Great guys, super knowledgable and really helpful.

So I had everything installed and double checked. New Wheel cylinders got rid of some annoying leaks and I adjusted the snails, which all went well. Brakes were firm, but I bit too much pedal travel for my liking. So I thought I would adjust the Brake Master cylinder piston rod to lift the bite point of the brake pedal.

My Work Shop Manual (version 4291) gives limited information on this subject. In fact the only reference is H18-5
“Check the freeplay in the piston rod; if it is less that 1/16 inch (1.6mm).
Elsewhere I had read a suggestion to adjust the piston rod to give yourself a higher brake pedal.

So I did what I thought felt about right for me and did a drive-way test. High brake pedal, firm brake feel with no
sponginess. So off for a short road test.

I live on a slight hill so headed up the hill, braking firmly at the intersection. All good, I thought. So around the next block which took me back to the my street, but now I was on top of the gentle hill.

The car rolled nicely so I coasted down the gentle hill. I started to gently apply the brakes to slow down a bit so I could turn into my driveway comfortably. I pushed harder as I came to my drive-way, and the brake pedal went to the floor!!! Fortunately my transmission brake works a treat, so that drew me to a stop, safely, just outside my garage.

After recovering from the shock, I suspected the Brake Master cylinder has somehow failed me, somehow.
I put the the vehicle onto to my 4 post hoist, got under and stripped the Brake Master cylinder of its internals in situ, a trick I learned a while back.
Seals were all good, no damage, no wear. Everything was in place and correct.

After an evenings contemplation, I realised what I had done wrong. I had “over shortened” the piston rod, which have moved he functional points of the two seals, so that the small but consistent pressure I was applying to the brake pedal while coasting down the hill, had allowed the fluid to creep past the pressure seal, so when I tried to apply more braking force, the pedal when to the floor, as there was no brake fluid in the pressure chamber to activate the brakes.

So the 1/16 inch (1.6mm) free-play in the piston rod is absolutely critical to the functionality of the brakes.
I hope this nasty experience helps someone else from making this same mistake.

Regards
Jeff

Jeff,

Thanks for the warning! Lucky escape for you, your Landie and your garage! If you have too much pedal travel with a firm feel, your shoes are too far from the drum. Jack it up and adjust them again. Driving around has probably bedded them in requiring a minor adjustment. Mike (5380)