Ask The Mechanic – 168 – Replacing the brake master cylinder

For this installment of The mechanic, we welcome a submission from Jonathan Bruton. You may recall his submission for issue TT166 concerning brake overhaul, this is the second chapter of that story.
Not long after I had put my tools away and given
myself a smug pat on the back for having successfully
installed new callipers and discs on the front wheels
of Mortimer Henderson (TT Issue 166), my ’73 Bay, I
happened to see a Facebook post from Nick Gillott
to the effect that the master cylinder also needed
replacing at regular intervals. The master cylinder, as
its name suggests, pushes brake fluid through the
lines to the slave cylinders at the wheels when you
hit the brake pedal, operating the brakes through
hydraulic pressure.
Anyway, I tried to ignore this unwelcome piece of
advice but could no longer do so when it became
obvious that the pedal was getting spongier by the
journey; when I finally got around to checking the
level of the reservoir, it had gone down quite sharply,
and I could see brake fluid dripping out of the hole
in the front pan beneath the pedal assemblies. So,
action was clearly needed.


Once the pan was removed, the first thing to do
was to locate the cylinder, which I had never looked
for or seen. As you would expect, it is bolted to the
frame beneath the brake pedal assembly, and the
brake pedal rod fits into it through a rubber boot,
which itself fits through a hole in the frame and is
designed to keep out dirt and debris. Two brake lines
lead away from it – one to a T-piece which then feeds
the front wheel brake assemblies, and the other to a
pressure equaliser bolted to the offside edge of the
frame, which feeds the rear brakes.
The main fluid reservoir crouches on it piggyback style and is attached via two nozzles that run
through rubber grommets. Finally, the brake light
switch screws in at the back (on my replacement
cylinder, there were two holes for the switch, and
a video I watched for the same job on an early Bay
showed two brake light switches, for reasons I’m not
clear about).


At first glance it was immediately apparent that all
was indeed not well. The boot was in shreds, and the
assembly was clearly leaking, presumably because
dirt had penetrated the seal. But replacing it looked
pretty straight forward, and I naively anticipated that
it’d be done in a single afternoon! It really needed
to be as well, because we only have one parking
space, which has the charger for our main car, a fully
electric Nissan Leaf, which we can’t use if it’s blocked
by a hulking great immobilised van! This has been a
point of friction between me and my long-suffering
partner in the past, but I blithely assured her that
there would be minimal disruption.
In this optimistic spirit, I ordered the replacement
part from JK and offered it up to make sure it was the
same as the one on the van, which it was. So now it
was a matter of whipping off the two 13 mm nuts
holding it on, unplugging the brake light switch and
undoing the two brake pipes, emptying the fluid
reservoir in the process. Yeah, right!
For some reason best known to themselves, VW had
opted for nuts and bolts rather than studs to hold
the cylinder on. Which would inevitably mean that
the whole bolt would just start rotating. Which both
of them did. With one of them, I could get a wrench
on the bolt head and get the nut off no problem.
The other one, however, was conveniently located
in a recess, making it impossible to access with a
wrench, so there was no way to hold the bolt still. In
the end I had to resort to a mechanical nut splitter to
remove the offending nut. With a bit of persuasion
by hammer, I was then able to loosen the cylinder
and start moving it backwards.
The next issue was with the two brake pipes. When
new, of course, the nut rotates freely around the
pipe. After 47 years of exposure to God knows what,
however, muck and corrosion do their worst, and
the nut sticks fast to the pipe. Once I’d been forced
to buy a new 11 mm wrench (inevitably, the only
wrench missing from my set was the one I needed), I
ended up doing what the guy in the early Bay video
had earnestly warned me I really didn’t want to do,
which was to shear both of the nuts right off. After
a few seconds of panic, however, I realised that both
sections of pipe were relatively short and could
easily be unbolted from the other end: at the abovementioned T-junction and the pressure equaliser.


Perhaps this kind of damage is more consequential
in an early Bay. Whatever, I then relaxed and let the
brake fluid drain out through the fractured pipe ends
into a handy receptacle below. My advice would be
to assume that these pipes are going to be toast and
simply order replacements when you order a new
cylinder; it’s no big deal.
So, having broken both pipes and removed the
retaining bolts, I took the cap off the brake light
switch and pulled the cylinder out, complete with
fluid reservoir. Now, this is attached to the secondary
reservoir in the cab by a length of plastic pipe held
in with two plastic hose clips, themselves secured by
two tiny cross-headed screws. These are a bit pesky
to reach, but I got the lower one out easily enough,
assuming I wouldn’t need to move the uppermost
one, and removed the whole assembly.
The reservoir plugs into the cylinder in two places, as
I said above, and it’s a very tight fit – which it needs
to be – so I had to use a screwdriver to exert some
leverage to get it off. No problem there. It was in
good nick, with no cracks or splits, so I could simply
reuse it. The new cylinder comes with the sealing
grommets, so you just have to use some elbow
grease to push the reservoir on. Just make sure you
get it the right way round! Once it was all in place,
I bolted the cylinder in place, having replaced both
nuts and bolts.
Annoyingly, I missed the delivery driver when he
came with the new brake pipes the following day.
That day being Friday, it meant that the van would
have to sit on the space until at least Monday. I
averted a charging-related roasting by offering to
take the Leaf up to the nearest charging station,
so harmony was restored. Monday came and
the eagerly awaited pipes with it. As they have a
diameter of 3/16 “, they’re very easy to bend without
kinking. The only issue here was that the length of
pipe that went to the pressure equaliser was only
just long enough, meaning that I had to carefully
plan the shortest possible distance.
Having removed the old pipes, it was then something
of an epic task to get the nuts to engage with the
threads at both ends – I would get one in place, only
to find that the other end simply wouldn’t oblige. In
the end, I had to loosen the cylinder body again, and,
after rather a lot of swearing, the nuts were finally in
place, and I could reattach the cylinder to the frame
and reinsert the brake pedal rod into the boot.
Surely it would now just be a simple matter of
reattaching the plastic pipe to the bottom reservoir,
refilling it with fresh fluid, and bleeding the brakes.
Ahem. Not quite.
To start with, there was the second hole for the
missing brake light switch. Not much point putting
fluid in for it simply to run out again through a
great big hole. As automotive bolt threads seem to

be narrower than their DIY counterparts, my local
hardware store was unable to provide a suitable
blank. Happily, they directed me to a garage round
the corner, and the chap there fished around until
he found a bolt with a nipple, which looked like it
came from a carburettor assembly, that had the right
thread and would do the job. Now, it would surely
all work.
With great lightness of heart, I tightened everything
up and started to refill the cab reservoir – only
to discover that the fluid was dripping out at the
bottom almost as fast as it was going in! Yes – it
was the hose. Leaking at both ends. Meaning that,
to investigate, I’d also have to undo the topmost
clamp, which was virtually impossible to reach
from underneath. Filing that away as a problem for
later, I replaced the pathetic little plastic clip at the
bottom end of the hose – where it joined the lower
reservoir – with a proper jubilee clip and tightened
it nice and snug. I then had the blindingly obvious
realisation that it would surely be possible to undo
the reservoir in the cab and lift it out to get access
to the clamp immediately below it. But I couldn’t see
how to release the reservoir. Fortunately, the Samba
came to the rescue, and I was soon undoing the two
little screws that held it in, which enabled me to lift
up the reservoir and shed light on the problem.
Sure enough, the hose at the top end was split, so I
trimmed it and replaced the plastic clip with another
metal pipe clamp. I also realised that the nozzle (I
can’t think of the proper word for the protruding part
the clamp attaches to!) and was supposed to have a
plastic sleeve around it to aid the seal, but this sleeve
was missing from both ends, so all I could do was
make sure the clamps were located on the slight
bulge in the nozzle and done up nice and tight.
And then – glory be! – the leak was finally sorted!
I filled her up and fetched my handy little Draper oneman bleeding kit, which is a bottle with a one-way
plastic hose that fits snugly over the bleed nipple
and doesn’t permit any backflow. When you’re lying
under the van, you can operate the brake pedal from
underneath and watch as the air bubbles shoot out
of the bleed nipple and disappear into the bottle,
to be replaced by a lovely golden bar of brake fluid,
which is a fine sight.
So, there it was. All done. Except that I couldn’t find
the cab fluid reservoir cap. Anywhere. I’m sure many
of you will know what it’s like not to be able to find
the tool you’ve just put down and to have to spend
ten minutes searching for it until you find it in your
pocket or somewhere. Anyway, as my frustration
and incredulity increased, I resorted to rummaging
through the recycling until I found the top of a
squash bottle which could be made to fit. Better
than nothing! Anyway, I could finally triumphantly
drive the bus off the parking space and swap it for
the Leaf, which I plugged in, thereby ensuring that
domestic harmony would continue without a ripple.
And then, there was the reservoir cap. Perched
on top of a wheelie bin, where I’d left it. Laughing
at me.
Jonathan Bruton