No, no pressure relief valve, it is likely the piston pots have corroded brake dust stuck on them and need a through clean. also check for pitting on the piston pots, if they are pitted they will also need replacing. (as you now won't have a perfect seal which could cause substantial issues, also the rough edges of the pitting may tear the new rubber seals, again resulting what could be a substantial issue)
If doing at home and not using the parts washer at work, order up a can or two of brake cleaner, a new tooth brush and red rubber Grease. (a tooth brush is good for rubbing away caked in brake dust / road grime on a piston pot sprayed with brake cleaner without scoring or damaging the surface of the piston pot.) also good for cleaning off the crud around the calliper and piston ports. ( if you don't clean the callipers using the works parts cleaner)
The red rubber grease is good for the seals and lubes them up for when you put the pistons back in.
Small g clamps and cardboard / thin slice of wood are good for pushing pistons back in to the calliper.
Undo the fluid reservoir, syphon out the fluid in it, (as you know taking care not to get any on the bike / paint work).
Put a piece of cardboard / slice of wood on the rear of the calliper and another piece / slice over the front / piston pot, slide a g clamp over them and start to tighten the clamp down gently (pushing the piston back into the calliper) do it slowly and keep an eye on the fluid resivour so it doesn't spill over. Once done throughly bleed the braking system.