Brake issue - master cylinder or brake booster?

Hi guys,

A while ago I introduced my litte nugget here, in case you missed it here it is :slight_smile:

In the last couple months I haven’t been able to work on it and now I’m picking it up again. I got the engine started and luckily it seems to be healthy (I got it from the junkyard blindly believing the seller it was in good condition).

The issue I’m having now are the brakes, I changed the stock Cuore ones for the Sirion big brakes with ABS sensors. After full assembly I took out the air from the system but noticed the pedal stayed spongey when rapidly pressing it while engines was off. When I checked the master cylinder it was leaking fluid onto the brake booster. I replaced it and bleed the brakes again, now the pedal feels a bit more solid when the engine is off.

When I turn the engine on though, the pedal goes straight onto the floor and it feels like it basically isn’t functioning. Also when it goes to the floor the RPM fly up and when pumping it’s like pumping up the revs. Since I replaced the master cylinder and it isn’t showing any (external) leaks, could it be the brake booster or would you suspect something else?

You’ve possibly done something majorly wrong. When you say you replaced the master cylinder, what was it replaced with? Did you replace the vacuum booster also? Same size one? Was there a check valve on the orig hose? Did you swap vacuum hoses? The only way the brakes are linked to the engine possibly revving is via the vacuum hose. With the throttle closed, how can you get more air into and engine and make if rev? We put a hole somewhere in the manifold and let more air in. Sounds like you are sucking more air into the engine via the vacuum hose. Check vacuum lines first. Once that is fixed you’ll get an idea if it still needs a bleed.

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When you replaced the master cylinder are you 100% sure the new one is exactly the same as the old one?
I had a problem with a MC leak on my sons L500 Mira and it was very difficult to find a new MC, I ended up finding a seal kit which finally fixed the leak.
There is a rubber seal between the MC and the booster, if that doesn’t seal properly you could have a vacuum leak which could explain the increased engine revs but the pedal still shouldn’t go to the floor.
What method did you use to bleed the brakes?
When I replace a MC I firstly bleed the MC by loosening the pipes at the MC, then start bleeding at the wheel furthest away from the MC then continue with the next closest etc.

Don’t know if it’s even possible on a Daihatsu but I have seen cars where its possible to fit the brake calipers on the wrong side which can result in the caliper bleeder being on the bottom instead of the top, if the bleeder is on the bottom it will be near impossible to bleed the brakes properly so check the location of the bleeders.

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I’m pretty sure it’s possible, so it’s worth a check

Hi Guys,

Thank you for all the support, I really appreciate it :smiley:

I double checked the master cylinder (from scrapyard same car), rubber seal mc & booster, the vacuum hose and the calipers. It turned out that the caliper bleeding screw was indeed on the bottom instead of the top, I switched them around, bleed the system again and now it’s working great! :muscle:


The calliper upside down is certainly the bleed and pedal feel issue. However, I can’t see how it can ever affect the idle issue.