Beest, there are rubber or plastic seals inside. What you'll find inside are felt washers (left), springs and rubber washers (right).
. As well as the pistons. MAKE SURE YOU DO NOT MIX UP THE PISTONS when you take them out. They are likely not numbered and are NOT identical, but easy to mix up. Follow best practice as in Zen and the Art of Motorcycle Maintenance.
Sent by thought transference