It is not your shower - it belongs to your landlord - if you repair it and it breaks or fails to work either immediately or at any time after your maintenance work your landlord could hold you liable for a new shower and the cost of installation.
Judging from the reluctance to have it serviced professionally I suspect he/she will extract maximum benefit from the situation.
Send him a letter and withold rent until it's been properly fixed - speak to your loacl authority - find somewhere else to live. Tell the landlord that if it's not fixed you will have it professionally repaired and send him/her the bill and take it out of the rent. Make a fuss.
If you still wish to go ahead and mess with it get a letter from the landlord absolving you from any future liability.
Good luck...
Peccavi
January 2010
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix baths and showers.