Brake servo causing brakes to bind

[Greta]

Although I can find several online threads on various piston head type forums referring to this - no one ever seems to come back and explain  the definitive cause or a permanent fix that works!

I'm now on my third servo - two of which have been brand new - one cheap chinese rimmer repro the final one an expensive original from Moss. .. and the problem persists. I don't want to bypass the servo.

It's fitted to my MKIV 1500 engined Spitfire and the servo sits in the battery box ( battery in the boot ) - The angle and mount position seem exactly the same as when every thing worked perfectly. I've serviced brake master cylinder too. Bleeding the brake line to the servo cylinder is all that is required to free the brakes. Sitting in the garage with the engine running and pumping the brakes doesn't seem to induce the problem - but a short run with say half a dozen decent brakes will cause the pedal to harden & the brakes to lock on!!

Please help - what do I do next?

[yorkshire_spam]

Sounds like something (PWDA, restrictor, master cylinder?) is causing a residual pressure in the brake line between the master and the servo, so the air valve/bypass isn't functioning as it should?

[Bitumen Boy]

I know you don't want to bypass the servo, but I'd be tempted to do so on a temporary basis just to be 100% sure the servo is causing the problem. 3 servos with the same fault strikes me as being more than usually unlucky.

[Greta]

Quoted from Bitumen Boy

I know you don't want to bypass the servo, but I'd be tempted to do so on a temporary basis just to be 100% sure the servo is causing the problem. 3 servos with the same fault strikes me as being more than usually unlucky.

Yes I see what you mean. Does anyone know of a brake line junction box with just two ports?  - of which I'd need two if I was to construct a short bypass without the need to replace the existing brake lines?

[poppyman]

they  used to be available, i used them many times to join pipes that had to be routed around subframes and such to save time if the pipes were ok.
if you have an old school garage near you they might have them?
if not just have a  short new pipe made with female fittings

[Greta]

http://www.lotuselan.net/forums/download/file.php?id=5904

OK can someone confirm that the connection of brake pipes should be as per the manual in the download above. Because if I'm interpreting the labels correctly the experienced triumph workshop that recently recommissioned my car have connected the pipes the wrong way round.

So to be clear the feed from the brake master cylinder SHOULD: -  ENTER the servo in the SIDE of the slave cylinder and LEAVE towards the three way connector to front and rear brakes from the END - of the servo.

Please someone confirm this and reassure me that making up new brake lines will finally solve my problem.

BTW disconnecting the vacuum pipe from the inlet manifold at the inline non return valve and plugging with an appropriate size nut seems to have confirmed that the rest of the brake system is OK.

[poppyman]

Quoted from Greta

http://www.lotuselan.net/forums/download/file.php?id=5904

OK can someone confirm that the connection of brake pipes should be as per the manual in the download above. Because if I'm interpreting the labels correctly the experienced triumph workshop that recently recommissioned my car have connected the pipes the wrong way round.

So to be clear the feed from the brake master cylinder SHOULD: -  ENTER the servo in the SIDE of the slave cylinder and LEAVE towards the three way connector to front and rear brakes from the END - of the servo.

Please someone confirm this and reassure me that making up new brake lines will finally solve my problem.

BTW disconnecting the vacuum pipe from the inlet manifold at the inline non return valve and plugging with an appropriate size nut seems to have confirmed that the rest of the brake system is OK.

that looks good to me, i would have fitted it the way it is shown in fig 4.
tony.

[heraldcoupe]

Quoted from Greta

I've serviced brake master cylinder too.

When you serviced the master cylinder, did you replace the main spring? Reason I ask is that I've had work hardened springs cause exactly this fault, gradually pressurising the brakes with each application of the pedal. The first time I found this the spring appeared to be physically perfect, however approximately one third of it's length had become work hardened; this section would not compress.
It was the last thing I changed...

Cheers,
Bill.

[Greta]

Quoted from heraldcoupe

When you serviced the master cylinder, did you replace the main spring? Reason I ask is that I've had work hardened springs cause exactly this fault, gradually pressurising the brakes with each application of the pedal. The first time I found this the spring appeared to be physically perfect, however approximately one third of it's length had become work hardened; this section would not compress.
It was the last thing I changed...

Cheers,
Bill.

Thanks Bill - what you describe is exactly the symptoms I have - but I'm beginning to believe there are multiple reasons. Not only are the brake lines connected wrongly but if I am interpreting the diagrams in the link the one way valve is also the wrong way round.

As my brain is now so scrambled please confirm that air should be allowed to flow towards the inlet manifold and not towards the servo!? ...it's easy to test the valve by blowing into it - but the arrow that should be on it isn't legible and the stamp that reads servo maybe printed upside down!

[url]http://www.mgb-stuff.org.uk/servo.htm[url] - describes more fully servo operation.

[url]http://www.triumph-spitfire.nl/servoimages.htm[url]. - useful site on servo dismantling & service.

[Nick Jones]

Air should be allowed to flow towards the inlet manifold.  The engine is sucking the air out of the servo.  If you are getting any servo assistance at all with the valve the wrong way around then the valve doesn't work properly anyway.

I think some remote servos are a bit fussy about the angle they are mounted at (supposed to be slightly nose up, where the nose is the outlet port) though I've never understood why.

Nick

[Pete Lewis]

its angles just to get the air out as there is no bleed on the cylinder

do check the brake pedal is returning fully and with foot off  you have some free slop on the master cyl pushrod
if there is no free play you build up pressure , undo a bleed nipple releases the pressure till a few applications later ,

things that affect the pedal return
  as Bill has posted
  badly set brake light switch
seized pedal pivot pins
or the servo diaphragm pushrod is binding in its hole thro the canister
weak spring on the air valve return

just some thoughts
Pete

[GT6 M]

If, and its a bigg IF, ye done what I did whenst repairing the MC, then brakes could bind,

wot I did, was mek an adjustable push rod for the MC
idea was t,get as much ..free play oot the system as possible
butt, just nipped it tooo farr, and the seal ont end of shaft inside the MC, blocked the fluid of frae returning

this aint wot ye done also is it,!!!

Note, its a well wuth wee mod, as the amount of play it can get rid of is Alottttt
so pedal stays at top  most of the time, even under hard brakin


M