When I got my 500SL over a year ago I replaced the 43 year old brake hoses for braided hoses but after I installed the new (HEL) hose I could not bleed the rear brake. The brake master, a replica PS12 with integrated reservoir was seized and the piston would not release no matter what. I always use Castrol Dot 5.1 brake fluid in all my cars and bikes. These replica brembos cannot be disassembled as they are pressed together. After looking endlessley for a rebuildable original I decided to buy another replica PS12. I think Officine08 and Stein-Dinse sell the same unit. Before I install this new MC I would like to know if anybody has experienced the same thing and if it can be caused by using DOT 5.1?
The ones from stein are PS15 castings but bored PS12 so the "nose" of the casting is a little longer than the original PS12. I have sold dozens of these and never had a problem. stick to Dot 4 on these old brembo parts for the sake of the master cylinder and the caliper seals. I doubt 5.1 would cause a problem but its not really needed in these old brake systems as i doubt you are boiling the fluid with the back brake
I use 5.1 in my 60's Fulvia and in both my 'modern' 90's cars. I have used it in all my vehicles since the early 2000's. Never had problems. Could 5.1 have caused the seizing of the new replacement PS 12 (chinese?) fake Brembo pump?
5.1 is hygroscopic which means it takes on water from the atmosphere, this rusts the master cylinder piston which then seizes in the piston bore. Ideally 5.1 should be replaced once a year. Hygroscopic nature:Unlike silicone-based DOT 5, DOT 5.1 is glycol-based and is designed to absorb moisture from the air to maintain a consistent boiling point. Moisture ingress:Water can enter the sealed brake system over time, despite its closed nature. Consequences of Water Contamination Reduced boiling point:The absorbed water lowers the fluid's boiling point. Vaporization:Under heavy braking, the water in the fluid can turn to steam (vaporize), creating compressible pockets of gas in the brake lines. Spongy pedal:The presence of these vapor pockets causes the brake pedal to feel soft or "spongy". Brake fade:This leads to a loss of braking effectiveness, also known as brake fade.
I bought this bike with a 6 months old replica PS12 on it filled with an, to me, unknown type of fluid. The rear brake functioned faultlessley but as the hoses where marked 1981 I felt the need to replace them. I replaced the brake line and filled it with Castrol 5.1. Almost immediately during the bleeding procedure the piston would not retract anymore. I removed it from the bike and tried to free the piston soaking it in Dot 4. Nothing helped. I left it in my ultrasonic cleaning tank also to no avail. with a pair of pliers you could pull back the piston but it was hopeless so I bought another one of presumably the same (chinese?) manufacturer.
In that case the seal on the piston has reacted with the brake fluid and swelled causing the piston the seize. I have seen this before.
Okay, I will fill the system with Dot4. Have to buy it first as all my stock is 5.1 Just for the record, the front brakes are filled with 5.1 and work flawlessley.
The units are definitely original Brembo but according to the owner where rebuild using rebuild kits from Officine08. Don’t know where they got them from.
