S: On a hot day, if you start to lose brakes (T25/KA-T)…
S: On a hot day, you start to lose brakes after boosting for a while. Brake pedal will get spongy and start to travel closer and closer to the floor as if there was a leak somewhere in the brake lines. Brakes come back to normal operation few hours later once car is cool again.
C: Very likely problem with Master Brake Cylinder heating up.
Here is what happens – usually occurs with boost past 9 psi. Please note that ballpark measurements over time were taken with infra red meter. With car boosting a lot especially during hot weather, manifold on T25 can reach close to 280 degrees C and on the underside of the turbo near wastegate / turbine close to 380 degrees (tested on non-water cooled t25).
As long as you are driving, the flow of air through the engine bay will keep the air moving preventing the hot parts impacting the Master Brake Cylinder (MBC). The problem often occurs once car stops and is either turned off or sits idling for a while – about 10 minutes later, you start car, drive off but notice the brakes are really weak and travel further to the floor. What happened was that due to the lack of moving air through the engine bay – the manifold and turbo started cooking your mbc – I’ve seen temperatures close to 98 degrees C on the MBC housing and 110 on strut bar above. Then when you let car sit for few hours, brakes return to normal operation once temperature of the MBC cools. I haven’t looked into why exactly the problem occurs inside the MBC – from looks of it oil rises towards top of MBC plastic container and seems like less oil / lubrication is available in the actual cylinder – if you push pedal on hot MBC, you can hear bit of scraping and lots of pedal travel indicates there is just no oil in the cylinder when you push on it.
I noticed that anytime the MBC housing reaches over 65 degrees, you will start to see degraded braking performance such as spongy pedal, etc.
Solution: Make a brake heat shield & replace fluid in MBC with synthetic fluid. Issue went away for me just by heat wrapping the MBC & just draining fluid from mbc & refilling with synthetic dot 4 fluid.