[Mgs] Clutch Master cylinder problem

Barney Gaylord barneymg at mgaguru.com
Sun Feb 3 15:58:13 MST 2008


At 01:40 PM 2/3/2008 -0800, Merl Rosenthal wrote:
>I replaced all hydraulic clutch components (clutch cylinder, slave 
>cylinder, rubber hose, etc) on my 1980 MGB about 11 months ago 
>during a ground up restoration.  I purchased the clutch cylinder 
>from Apple Hydraulic who stated I could use synthetic brake fluid 
>since I replaced all components.  I just checked the fluid .... and 
>wiped a Q tip inside the reservoir and found a black looking 
>substance on the end of the tip and a small glob floating in the chamber.  ....

I had a similar experience with my MGA several years ago, but the 
other way around, when switching from DOT-5 to DOT-4 fluid.  I had 
been using DOT-5 fluid for 12 years and 130,000 miles with no 
probelm.  I thought it would be good preventative maintenance to 
change out all of the rubber parts in the hydraulic 
system.  Following the great brake fluid argments, I took the 
opportunity to switch to DOT-4 fluid for a long term test while 
installing all new rubber bits.  I drained and flushed the system 
with alcohol and blew it out dry during disassembly, then honed the 
cylinders and installed all new cylinder packing kits and hoses.

In 6 months time the fluid turned dark and brackish looking with 
little rubber bits included.  I flushed through all fresh fluid, but 
it did the same again in another 6 months, so I flushed it again.  At 
18 months the master cylinder started to leak, and the DOT-4 fluid 
was damaging paint on the heater shelf.  So I did the whole job 
again, drained and flushed with alcohol, blown our dry, repacked all 
cylinders with new rubber seals, and replaced all hoses.  I then went 
back to using DOT-5 fluid for no other reason than it doesn't eat 
paint if it leaks.  Since then, 6 years and 40,000 miles with no problem.

I would not blame DOT-4 fluid for my problem any more than you should 
blame DOT-5 fluid for your problem.  I suppose the root cause of the 
problem is bad rubber seals that deteriorate when exposed to brake 
fluid.  We have had sporadic rashes of bad rubber parts of all sorts 
over the past several years.  The problems come and go.

I wouldn't try to guess what rubber parts in your system are failing, 
cound be cylinder seal cups on either cylinder or the hose.  Silicone 
fluid should be a natural presevative for all sorts of elastomer 
parts.  When you remove the cylinder seal cups for inspection they 
should look almost like new with a firm smooth shiney surface and all 
the little molding numbers clearly visible.  Mine looked that good 
after 12 years of service with DOT-5 fluid.  If the rubber surface 
appears dull and porus, or it tends to rub off when you rub it 
between your fingers, you have found the bad pat.  If it turns out to 
be bad cylinder packing seals, Apple Hydraulics should have a two 
year warrantee on their rebuild cylinders.  It might be easier and 
quicker to replace the packing parts yourself rather than sending the 
cylinders back for warrantee exchange.

Barney Gaylord
1958 MGA with an attitude
http://MGAguru.com


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