The brake pedal would slowly sink to the floor after several seconds of pressure. There was no fluid leak and my mechanic and I concluded that the master cylinder had an internal seal failure. We replaced the master cylinder and the vacuum servo for good measure (welsh had an irresistible sale). Brakes worked great through the summer but after a couple hundred miles the same problem has returned. The brake pedal is hard with great vacuum assist for several seconds then the pedal slowly sinks to the floor with loss of all breaking power. There is no loss of fluid. Reservoirs are full. What am I missing?

 

Greetings from down under. My advice comes bound up with several provisos -

• brake fluid is hygroscopic (attracts moisture) so components tend to corrode from inside out;
• replacing m/c seals was a good call, but what was condition of bore - most need honing at least, many need relining, otherwise new seals become dead seals very quickly;
• finally, bleeding compressible air from ALL lines is critical.

None of these comments are a slap either for you or your mechanic, but so often experienced here, that they are your priorities to check.

Best wishes,

Ken

 

Thanks Ken, the replacement master cylinder was new as was the vacuum servo. All calipers and lines were checked and old fluid flushed and replaced with new. Everything worked great for a few hundred miles and then the problem recurred. The etype has a complicated system where pressurized fluid pushes out of the master cylinder into another cylinder mounted on the end if the vacuum booster. This second cylinder then pushes fluid to the calipers with increased pressure from the vacuum system. I think the problem is in one of these cylinders but I don't know which one. Nor do I know why new parts would fail so quickly. I would really appreciate your thoughts. A close friend of mine lives in Australia. He has a 1930 rolls Royce that he manages to keep running so I know you guys are good keeping these old cars on the road. thanks again.
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OK, no leaks so one can assume the calipers are all OK.

But have you checked every one including the hard-to-see rears ? I suggest you check the rear calipers again, as it is these that tend to be neglected due to their position. I had a sinking pedal on an old XJ6 Series 3 in the mid-nineties and it eventually got to Brown-Stained Underwear time descending a long hill !! An examination showed it was the rear calipers. No accident, BTW, as I pumped the pedal.

Another thing is were the replacements new-old stock or fresh from the manufacturer ?

The brake booster has a valve that varies the booster pressure depending on how much you're pushing the pedal. If fluid gets past the seal on this, it is drawn into the inlet manifold so won't be seen as leaking, but shows itself as white smoke in the exhaust.

Anyway, that's my two cents worth !!

 

Thanks Fraser, the rear calipers are ok. You ask a very good question. How old are the "new" parts I bought. I'll ask the seller. Also, your right, if the fluid pushes past the seals in the brake booster it would get pulled into the intake manifold and burned giving white smoke. Once I thought I saw some white smoke when I stepped on the brakes but all the other times no. Also if fluid was lost to the intake manifolds the reservors would be low and mine are full. I see two options. Pull off both the master and the booster cylinders, dismantle them and rebuild the one with the bad seals But at $100/hr labor it would be cheaper to pull both off, throw them away and install new cylinders again. I just have the feeling that something else is causing this failure and if I replace the parts now I'll have the same problem again by next summer.

 

Hmm.I would not replace both items together this time. Rebuild the new one you have and also carefully inspect the cylinder bores. Problem nowadays is the quality of parts, if sourced from other than original makers is the process controls are not up to snuff, so bad stuff gets sent out.

I'm no totally up-to-speed on the Series 3 E-type, but with no loss of fluid it certainly does point to the master cylinder or the booster.

 

Thanks, into the shop tomorrow. I will try to post pictures of any damaged or worn parts that are found.

 

Thanks, into the shop tomorrow. I will try to post pictures of any damaged or worn parts that are found.

 

If it's any consolation, we have had similar problems here in the UK with MG Midget and Austin Healey Sprite parts. Fortunately my 1977 Midget is virtually all original, so has been reliable since purchase in 2010.