Author Topic: MK5 Front Brake failure and other front mudguard issues  (Read 341 times)

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Offline Hamwic

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Hi All,

I recently took on another MK5 that had been standing a while with carburettor issues. Having swapped out the badly worn and corroded AMALs for a pair of Chinese 19mm Dellortos (25 a pop fully jetted) I set off for a ride. The marginal front brake fairly soon faded away to nothing, and so serious investigation became essential.

It seems that back when it was made, Our Boy whose tedious job it was to fettle the link forgings had got a bit carried away, and the outer radius where the seal is supposed to bear, had been ground under size on the brake side. The seal lip was actually over the edge, and where the bike had been standing for so long, the inner seal lip had given up. Oil had been finding its way down the link, and quietly filling up the brake drum. The shoes oozed oil when squeezed, and given the lack of wear or scoring in the drum, I would say it's been like this a long time.

The link faces both sides were badly scored, so I have refaced these, and fitted shims as per recommendations elsewhere on this forum, but the brake side one has to be a bit over-size on diameter to give the seal lip something to bear against. It does seem that some oil had actually been coming down the splines, but a copious spread of water pump grease will hopefully prevent that happening again. A breather hole has been drilled in the filler screw, more in the hope of letting in dirt and moisture, which is obviously essential  :lol:

The small drain channel in the brake plate has been deepened a little, and a drain hole drilled from the felt groove down to the channel. Any excess oil getting in will hopefully find its way out again before it gets to the brake shoes, once again an optimistic view.

Turning my attention to the wayward and sloppy mudguard mounting, a fair bit of wear was to be found in all the usual places. Once again the recommendations on this forum have been followed, including beefing up the stays with through tubes where the 2BA bolts go through to restore the correct separation of the mounting plates.

It's interesting to query the designer's intention of this collection of parts. Was it the intention that the bronze bushes rotate on the central steel axes or on the link eye? It seems that the brake side rotates on the steel sleeve on the brake shoe pivot, but the other side is trapped by the side plates and rotates within the link eye.

A badly worn Offside Mounting Bracket (Item 10 on your hymn sheets) was also attended to, This was done by over-boring the holes using a step drill to a convenient new diameter, and making up bronze bushes to size them back to suit. These were soldered in, the wheel nut bearing being flanged out to about 10 thou shy of the space available on the wheel nut when fitted. Trying to arrange some lubrication for this, I realised that the wheel nut itself has been conveniently provided with a counter bore that is the the tapping size for a 3/8BSP plug. This, fitted with a grease nipple and some suitable lubrication ports, will allow a spot of grease (or six) to find its way onto the bearing face without recourse to dismantling. I should have drilled the holes adjacent to the peaks of the hex, but maybe next time?

There is little wear on the brake side plate - presumably due to the unintentional oiling it's been receiving all this time.

So now I'm just waiting on the re-lined brake shoes to come back, and hopefully stopping without drama slightly better than a veteran will again be possible. A bit of peace and quiet from the mudguard will add to the joy - less is certainly more!

Cheers for now
Doug

Offline eddie

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Re: MK5 Front Brake failure and other front mudguard issues
« Reply #1 on: 16 Jul 2022 at 15:17 »
Doug,
         I have never been able to understand why the Mk4/5 mudguard stay brackets have a 5/8" dia hole for the reaction link bearing. The whole set up would be more rigid if the earlier Mk3 short bearing sleeve was used and the holes reduced to 7/16". The 2 plates and the sleeve could then be clamped up - thus eliminating the slack in that bearing. With the whole assembly clamped to the brakeplate, it would also reduce the hammering effect on the bottom location. This modification - along with your widened bearing on the offside - should eliminate all the slack in the mudguard mounting brackets.
    Regarding the efficiency of the front brake - I have found it advantageous to ease out the hole in the centre of the 'top hat' bearing, then get an assistant to hold the front brake on whilst the front wheel spindle is tightened - thus centralising the brake plate to the drum.

  Regards,
               Eddie.

Offline Hamwic

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Re: MK5 Front Brake failure and other front mudguard issues
« Reply #2 on: 17 Jul 2022 at 10:49 »
Thanks for the reply Eddie - yes that would have been a better set-up.

I have attached a picture of the link/seal area. After taking 0.5mm off the seal area, this has been replaced by a 0.5 shim, so we are back at the "as supplied" dimensions. You can see that the seal face area was compromised from day 1. It also seems like a lot of "squash" on the seal as originally fitted. The cup is hard against the fork boss as far as I can tell. Plenty of greases applied at the fitting stage, but perhaps there are better solutions these days?

Cheers for now
Doug