stuart pump problem

hi everyone.
just signed up. love this forum, actually fixed my heating by reading help on here. couldn't get it fixed before. so i said i'd sign up and see if i can get my shower pump fixed too and maybe i can help others out some where.
any way have a stuart pump, ST55 3.5 bar. it's been working fine seen we moved into a new house 6 years ago. but over the last few years the pressure has been getting less. so i asked a plumber for advice. he said to pull off the 4 pipes going into the pump and check for blocks. so i did this and found a bit but not too much in the cold inlet. i put it back together and it ran a bit better (nothing like it was when we moved in)for a few minutes then it started going on and off. if you turn the water in the shower to full hot it works but as soon as you turn it back to warm enought to use, it starts turning on and off. if you leave it for ages it works for about 15- 20 seconds then starts playing up.
i guess its an air lock or something? but i'm not a plumber so don't know what to do.
thanks for any help
noel

Hi, I have found the Stuart & Turner technical help line very useful.

01491 572655

They can diagnose a problem over the phone and send parts in the post.

There is a long number on the top of the pump with indicates when it was made, I have found they normally last approx. 10 years.

If you tell us the number we can tell you how old it is.

Andy

There are "shuttles" which the flow moves, in the pipe. They have magnets in which close external contacts to do the on-off switching. Yours are scaled, up, blocked, split, or similar. Take it apart and have a look.

Pumps like these aren't generally known for the performance dropping over time, so I suspect something else has changed.

These are rather old-style ones, you can see one has let water in, and rusted and split, so it went sticky:

thanks guys.
the numbers are, S/NO 1410399/4302/W31861
part code 14-50-44238
i will pull it apart again and have a look, where are the shuttles?
noel

Your pump was made in March 1999. You are better off buying a new pump. (this is my opnion, and what I would do)

If you start replacing parts, you don't know what will go next.
It will be more money, hassle and more of your time.

Andy

thanks andy. it seems old enough. wish i could buy a new one
my problem is that i'm not getting any work these days (tiler)
and don't have any spare cash!
i took the whole pump apart and checked every thing. nothing looked bad.
i cleaned out any bits of dirt i found. but still the same. its definitly something to do with the cold water. as i said it was working ok just the pressure wasn't great.but i can live with that. it's only started the on off thing after i turned all the valuves off to clean it out.but i had a shower today. its stoped doing the on off thing and if i turn up the cold to full the water comes out quite hot but just about bearable for a shower.its not much more than a trickle.it's like it's not getting enough cold water.if i turn it to 50/50 cold/hot it pumps quite nicely but boiling water comes out and if i turn it to full hot it blasts out but at the same temp as lava.
help