The Moss Motoring Issue 1 2020 lists a brake master cylinder for a Mk 1 with the part number C20526 which is the part number for a clutch master cylinder. The brake master cylinder internals involve by-pass and feed ports and a check valve that the clutch master cylinder does not have. Although the external appearance of the two master cylinders looks to be the same, the internals are quite different. Would not this difference impair the brake action? Would the difference pose any danger?
Hi, on drum brake mk1’s the only difference between clutch and brake master cylinders is the check valve on the outlet of the brake cylinder. According the service manual the purpose is for making brake system bleeding easier. I have a new clutch cylinder ready to install as brake master cylinder on my car but the original is still working fine. Again, this is only valid for drum brake mk1’s.
Figure 7 on page L.10 of the Mk 1 Factory Repair Manual clearly shows and discusses the by-pass and feed ports on the brake master cylinder which the clutch master cylinder does not have. And this is for drum brake cars.
In page E8 it is stated that the only difference is the check valve on the outlet pressure line. I can confirm this as I recently resealed both cylinders on my car and all internals are identical except for the additional check valve on the brake m/c. As the original cylinder with check valve is no longer possible to buy it is definitely worth a try.
The statement on page E8 is wrong. It is true that the “internals” of the clutch and brake master cylinders are the same except for the brake check valve. This is illustrated by Figure 5 on page E8 and Figure 6 on page L9. One must examine Figure 7 on page L10 as I stated explicitly. It is not the “internals” that make the difference, it is the small holes drilled in the brake cylinder walls as shown on this Figure 7 where the holes are clearly labeled.
To confirm I just now checked my old clutch master cylinder. It has a small bypass hole about 5mm above the feed port. Difficult to get a good picture though.
Figure 7 page L10 clearly shows two holes which are labeled.
The small bypass port is always present regardless of clutch or brake cylinder. My original clutch cylinder has it and I also checked my spare aftermarket SNG clutch cylinder which also has the bypass. The function of the cylinders are depending on the bypass port to make sure the system is depressurised when the piston is fully retracted.
How about scanning a picture or two for rest of us to know what You’re talking about ? 
I just talked to a sales representative at SNG Barratt. He said that the clutch master cylinder does not have any holes drilled through its cylinder walls that connect with the fluid reservoir.
Ok, I would say he does not know the details of what they are selling in this case. I got mine from them about 3weeks ago and it has one big and a smaller hole 5mm above. Looks very much like a straight copy of the original brake master cylinder
but lacking the valve on the top. If I were you I would buy one with peace in mind.
Br svante
Thanks Svante,
A couple of questions: At which branch store of SNG Barratt did you buy you clutch cylinder? Is there any difference between the inside diameters of the clutch and brake master cylinders?
Lowell
PS: You have a very distinguished given name. Svante Arrhenius
won the Nobel Prize. Oskar Klein who was his student had a daughter who married a friend of mine.
Someone pointed out to me that the discussion in the repair manual about how the clutch master cylinder operates has this statement:
However, there is no mention of the second “feed port” that the brake cylinder has. So are you certain that there are two holes?
I got curious after seeing your posts which is why I checked both my original cylinders as well as the nicely made replacement I got from SNG UK. They are identically drilled internally with same dimensions, but it is quite hard to see especially the upper small drilling and even more difficult to take a decent picture of it…
I think the service manual confuses things a little by having different names on the small drilled hole of the clutch and brake master cylinders. Second feed port or bypass port is actually same thing. Could be that the manual had different authors for different chapters… if you think about the function, the cylinder will need two ports to work as intended.
Regards svante
I think that you are correct.