Hard brake pedal - too hard

[deschodt]

At some point I had MC issues which devolved into booster issues while at the mechanic's. I ended up with a good used booster (#3 as I recall, mine was bad, another replacement was bad, finally got one that worked) and a Tii master cylinder. My car is NOT a Tii but I believe is has rear brakes from a later car.  

 

Pedal feel used to be perfect, boosted but you could push the pedal and feel what you needed to feel and brake well...

Now, the pedal is too hard - there's some boosting (because of the booster failure I know what zero boost is like and that's not zero) - light braking feels normal, but it's a really hard pedal after the initial push for more serious braking... Locking wheels is possible ( I tried) but you have to be motivated and seems to me the effort required is abnormal...

 

Question is: I'm not sure why the mechanic put a Tii M/C in my car. Could that alone explain the poor pedal feel and harder force required? OR, is that OK and not all that significant and my booster is failing again ? Any simple test I can do with determine which is guilty, or common knowledge ?  Thanks ! 

Greg.

[TobyB]

Yup. that could do it. 

Tii m/c is quite a bit larger for the larger tii calipers.

That will give you less pad pressure for the same pedal pressure.

 

You could use vented Volvo calipers in front (with late e21 hubs and 77 e21 rotors)

to split the difference...

 

t

[Schnellvintage]

Did your mechanic replace the booster hose(s)? Check to make sure the vacuum check valve is installed properly the arrow should point in the direction toward the intake manifold. Not the booster. Also did he/she use the correct hose, so it doesn’t collapse under vacuum.?


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[Tommy]

Since difference from stock to tii caliper is so big in piston size, 34 to 40mm, I think it's dimensioning issue. That makes 38% more pedal force than tii from the hydraulics only, difference from rotor size adds some. Volvo calipers with 38mm pistons would bring the number to 11%.

[deschodt]

I see, so upgrade the brakes or reduce the M/C... Thanks for confirming !