70 cuda power disc brakes are too sensitive

[garycuda]

I just finish putting my 1970 AAR Cuda together. The car was not complete when  I purchase it. It had the brake and clutch pedals with the main bracket. I bought a used brake booster, reman master cylinder, used swivel assembly. The brakes are way too sensitive. If you push a little to hard, the brakes with lock up. Causing the tires to squeal.  I also installed used brake switch valve, metering valve and proportioning valve.

[Chryco Psycho]

a small bore master cyl may help giving more travel & modulation

[garycuda]

I tried to get a 1970 reman master cylinder with power disc. The local parts stores would get a master with the same size reservoirs. So I ended up getting a 1971 Master that  has the larger size reservoir for disc brakes. The bore size may larger then the 70 master. I will have to check that out.

[garycuda]

Chryco, The 1970 Cuda service manual says the master cylinder has a 1 1/8" dia bore. The 1971 manual says that the M C has a 1 1/32"  bore. The one on my Cuda now is from NAPA and they say the bore is 1 1/32" bore. Would that mean you would move less fluid as you push the pedal?  Is there an adjustment on the push rod from the booster to the pedal assy.

[Chryco Psycho]

yes the smaller bore will take more travel to move the same fluid amount , better modualtion , no adjustment between the pedal & booster , just between the booster & master .

[garycuda]

Could the rod between the booster and M C cause my locking up of the front brakes.

[Chryco Psycho]

No , the pedal could drop to low if the rod was too short  or the brakes could lock on if the pressure could not release back into the master if the rod is too long .
 It seems that the booster may be too efficient , the bore your Master cyl  has should be slightly less sensitive being smaller .

[garycuda]

Chryco             Could any of the brake switch, metering or proportioning valves cause this sensetive brake problem? Do you have any ideas that I should look in to to resolve this promblem.

[Chryco Psycho]

Check with a braking specialist like Baer , or Wilwood .
 IMO no , the proportioning valve balances pressure between the front & rear braking systems , the front brakes do 70% of the work so you have to reduce pressure to the rear system so the rear do not lock up most of the time , this would not cause an over sensitivity to the front brakes . most of the cars I have built are the other way around with excessive pressure needed to get the fronts up to the verge of locking .
 Others here might have a better idea how to reduce sensitivity , but you have a smaller master bore which is the first place I look to change the feel of the braking .
 Neil

[dlayden]

I have the same problem with a 1970 340 cuda. Installed all new brake parts ( fresh restoration) since changed both master cyl and new booster still way too sensitive. Did you find any cures?

[b-bak73]

you might have lost the reaction dampner inside the booster,its a rubber disc that sits in a cup on the end of the push rod coming out of your booster at the master cylinder end,if for some reason the push rod was pulled forward it can drop out and fall inside the booster,without it your brakes will be really sensitive and come on way too strong,its job is to dampen pedal pressure and apply braking softly,worth checking out before you buy any new parts.hope this helps.