I had removed Coolant hoses and capped them off several years ago. Two of the rubber caps experience the highest heat and pressure. As a result these 2 fail periodically. I have a stockpile of size 15 rubber caps and here is how I replaced the worn ones. All I needed for this job was a flatblade screwdriver.

Step 1: Cap #1.

Here is the first one I will tackle. It feels soft at the end and this is a sign that it will eventually burst. The first step is to remove the hose clamp holding it in place.

Step 2: Remove the Old Cap.

Twisting the old cap off the nipple, you can easily see the new and old side by side.

Step 3: Installing New Cap #1.

Wiggling the new cap onto the nipple, I put back the hose clamp and tightened securely.

Step 4: Cap #2.

This cap is under the distributor. Again the hose clamp has to come off first.

Step 5: Removal and Replacement.

Twisting the cap off and quickly putting on the new cap to reduce Coolant wastage. I used the handle of a hammer to push the cap fully onto the nipple. Hose clamp back on and tightened, the job is complete! Another inspection will be done in 3 months.