surely you have a sturdy newel post?
when you say weak walls, do you mean crumbly?
Get some brown plasplugs, a masonry drill, some 2-inch screws, a tube of no-more-nails or own-brand equivalent at a tenth the price, a vacuum cleaner with a hose, a squirty spray bottle or houseplant mister, and a damp sponge.
Mark the wall where the screw has to go. Identify that there are no switches or sockets directly above or below, or horizontally to the side.
Drill a hole that is long enough and wide enough for the plug and the screw to go all the way in. For the moment, it doesn't matter if it's loose.
Use the vac hose to suck the dust and grit out. Use the squirter to wash the remaining dust and grit out and immediately sponge away. Verify that the hole really is where you want it. Verify that the screws are long enough that, even after passing through the thing you want to fix, and the plaster (which may be an inch thick in an old house) they wil still go an inch into brickwork. Wash out the sponge.
Go and make some tea while the water dries up or soaks in.
Squirt the adhesive into the hole, starting with the nozzle dep inside so there is no air bubble at the back. Withdraw the nozzle as it fills. Push the plasplug deep into the soft adhesive, using a screw as a handle, so that it is at least an inch into the brickwork. Verify that the screw is still in the right place. Remove the screw if it will come out without distubing the plug. Smooth off the adhesive with a wet finger and sponge the wall clean.
Leave it overnight.
The next day you can drive the screws into the plugs and they will be held firmly in the adhesive which will spread the load and prevent the hole crumbling.
Also works for picture rails, kitchen cabinets etc.