Hi,

Have a 75 850 ES.

Got the bike back from a VERY reputable UK classic bike restorer (I live in Ireland). They gave the bike a once over & did a few updates for me.

The rear brake pedal has no resistance. I've just blead the brake using a vacuum tool which sucks the oil through the bleed nipple on the caliper. Seems fine, but the pedal just pushes down with no resistance.

What seems to have happened is that the plunger in the cylinder seems to be stuck fully in. When the seals are shot in the cylinder can this happen?

The UK company has just said the brake when fine when it left their workshop, I have no reason to doubt them but I'm now trying to figure out whats wrong.

It looks like I need to strip the master cylinder.

Before I do this is there something I'm missing?

Thanks in advance.

Kevin

The only alternative I can think of would be if some air was still trapped in the system somewhere?

HI L.A.B.

Thanks for the reply.

I know the system is quite simple, plunger pushes oil down line, calipar pistons push pads against brake!

I've used a vacuum pump to bleed the system, I filled the resovoir 3 times & no air was coming from the bleed nipple, just clean brake fluid.

It looks like the piston in the master cylinder is stuck all the way in. The rear brake is NOT binding.

Looks like I have another 'project'

I know that air can sometimes be trapped in the hose as the hose rises to a higher level than the caliper, so it can help if the hose is temporarily tied down to the swinging arm whilst bleeding, as then any trapped air bubbles should rise toward the caliper.

However I think it would be worthwhile rebuilding the rear master cylinder as the piston could have stuck. Or maybe the primary cup has split or worn severely? Also the piston return spring can become badly weakened by corrosion.

When the new or repaired master cylinder assembly is refitted to the casting it must be screwed in an by exact number of turns. If it is screwed in even one turn too many then the brake can drag or even lock on completely.

Also the position of the pushrod nut and locknut is critical so should be checked.

Hi L.A.B.

Many thanks for the info.

I'll try to bleed the system again with the line tied down if this does not work I'll do a complete strip down of the master cylinder & see what condition it is in.

Once again many thanks for the master cylinder rebuild info.


Kevin

Hi Kevin,

I'm with Les. Buy an MC rebuild kit and, if you're flush, a stainless piston kit for the caliper as well. In addition to Les' document, you can find an excellent rebuild tutorial on the Old Brits website. If you're working with original brake lines on your mk3, you really ought to replace the line; your ability to stop is depending on 35-year-old rubber. If you find that the rear MC cylinder is too corroded to use, Al Miles has stainless replacements.

I reckon you have sucked the piston as far as it will go & got stuck. If you bled it ye good old fashioned way, it would have been alright. They can stick if you push them right in.

Flo wrote:I reckon you have sucked the piston as far as it will go & got stuck. If you bled it ye good old fashioned way, it would have been alright. They can stick if you push them right in.

Hi Flo,

Thanks for your reply.

The, I think, stuck piston happened first before I used the vacuum pump, the 'sucking' did not cause the problem, the problem was already there.

I have the complete rear brake system now removed, calipar, lines & master cylinder.

I'll strip everything down & replace all the seals, I want to make sure my brakes work

Thanks to everybody for all your help.

Kevin

click wrote:The, I think, stuck piston happened first before I used the vacuum pump, the 'sucking' did not cause the problem, the problem was already there.

If the piston is found to be stuck, then I suggest you check the pushrod nut is set the correct distance away from the housing (see the instruction sheet I posted)? As the nut is a limit stop which should prevent the piston from being pushed too far into the cylinder. If the brake has been correctly built and bled, and with the nut adjusted to the correct clearance, then the nut should not come into contact with the housing when maximum braking force applied to the lever.

L.A.B. wrote:

click wrote:The, I think, stuck piston happened first before I used the vacuum pump, the 'sucking' did not cause the problem, the problem was already there.

If the piston is found to be stuck, then I suggest you check the pushrod nut is set the correct distance away from the housing (see the instruction sheet I posted)? As the nut is a limit stop which should prevent the piston from being pushed too far into the cylinder. If the brake has been correctly built and bled, and with the nut adjusted to the correct clearance, then the nut should not come into contact with the housing when maximum braking force applied to the lever.

Hi L.A.B.,

Many thanks for the extra info. I'll post an update when I have it, hopefully, all rebuilt & sorted.

More anon.

Kevin

rick in seattle wrote:you can find an excellent rebuild tutorial on the Old Brits website.

Indeed, however it contains one piece of information which I'm not sure is correct-which I have underlined;



quote:
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"Fifth, assemble the inside of the cylinder. Push the cup with the larger hole in the center (the other cup is not used) on to the piston with the flat top against the end of the piston as shown in the following picture. Push the valve into the end of the spring. Insert the spring into the cylinder with the valve going in first. Insert the solid rubber cup into the cylinder with the flat side facing out and the cup side against the spring. Place the two washers on top of the rubber cup with the flat washer against the cup. I have seen two spring washers on some cylinders, but the kit comes with one flat and one spring washer. I don't think it matters which direction the spring washer is inserted, just that it is against the piston end with the three holes. Insert the piston with the end with the three holes going in first. Lock the piston in place with the circlip. Place the rubber end cover on the end of the cylinder."
________________________________________

http://www.oldbritts.com/14_064244r.html




I have only ever found the thinner curved washer in that position in both Triumph and Norton master cylinders, and, as I recall, no (AP)Lockheed, Triumph or Norton exploded master cylinder diagram I have seen has shown more than the one washer?


As far as I know, the thicker washer does not belong with that piston assembly, apparently it belongs with a different design of master cylinder piston which also used a shallower ring type primary cup, however both of these extra components are generally included in the AP Lockheed master cylinder rebuild kits.