Replace a worn diaphragm inside your toilet's siphon to fix a weak or failing flush, without buying a new cistern.
SD-01Turn off the water and empty the cisternTurn off the isolation valve on the pipe feeding the cistern, or the main stopcock if there is no isolation valve. Flush the toilet to empty most of the water, holding the handle up to drain as much as possible. Soak up the last of the water in the base of the cistern with a sponge or old cloth.
SD-02Disconnect the water supply pipeUndo the tap connector nut where the supply pipe meets the underside of the cistern, using an adjustable spanner. Have a towel or bowl ready to catch any water left in the pipe. Move the pipe aside so it cannot drip into the cistern while you work.
SD-03Lift the cistern lid and free the flush linkageLift the cistern lid straight up and set it aside somewhere safe. Unclip or unhook the link between the flush handle and the siphon lift rod - most simply lift off. On push-button cisterns, unclip the plastic cable from the top of the siphon instead.
SD-04Remove the siphon from the cisternUndo the large plastic retaining nut on the underside of the cistern that holds the siphon in place - it is usually hand-tight, or needs a light grip with pliers. Lift the whole siphon assembly up and out through the top of the cistern. Tip any remaining water into a bucket.
SD-05Open the siphon and take out the old diaphragmTwist or unclip the siphon body - older designs split into two halves, newer ones have a twist-lock cap. Pull out the plastic diaphragm plate, which will likely be cracked, torn or stiff with age. Note which way round it sits so the new one goes in the same way.
SD-06Fit the new diaphragmMatch the new diaphragm to the make of your siphon - take the old one to a plumbers' merchant, or check the brand name stamped on the siphon body if unsure. Fit it the same way round as the old one, usually with the flap facing down towards the flush pipe. Clip or twist the siphon body back together until it is fully seated.
SD-07Refit the siphon and reconnect everythingLower the siphon back into the cistern and screw the retaining nut back on by hand until snug - do not overtighten the plastic thread. Reconnect the flush handle linkage or push-button cable to the top of the siphon. Reconnect the water supply pipe to the underside of the cistern and tighten the tap connector nut with the spanner.
SD-08Turn the water back on and test the flushOpen the isolation valve or stopcock fully and let the cistern fill. Flush the toilet a few times and check that the flush works properly and stops fully once finished. Check underneath and around the cistern for drips at the tap connector and retaining nut.
SD-09Adjust the water level and tidy upCheck the water level sits at the mark inside the cistern, usually about 10mm below the overflow pipe, and adjust the float valve arm or screw if needed. Refit the cistern lid carefully. Wipe up any spilled water and dispose of the old diaphragm.
Some links may earn us a small commission at no extra cost to you.