Hello TC I have been chasing a hard brake pedal for almost 9 months please help!.This is what I have come up with, looking for advise please! OK I have a 1972 SS BB Auto I purchased a master booster combo from OPG # LBS68-72. The master it came with was a 1969 or older style. So I orderd a new AC Delco Master for my 1972 the correct style.I also at the same time rebuilt my motor 402 30 over Comp 270H cam 9.6:1. and of course installed my new booster master combo.Ever since then I have had a hard brake pedal my engine produces at idle 12 in Hg vacuum. Not enough say OPG so I purchases a vacuum pump from power master brakes it now has 17-21 in hg vacuum still have hard pedal. New Master, booster, wheel cylinders, calipers proportion valve and hoses.Bled through a gal of fluid. Still have hard pedal poor braking.So my question for you guys could the master bore size be a miss match between master & booster. My new correct in apperance master has a bore size of 1 1/8 in. The master it came with I sent back as a core. graemlins/clonk.gif Could it have been a 1 in bore :confused: Could the booster require a 1 in bore? What is the bore size on the 1969 & older masters? Could this be why they come with the incorret appering masters? Sorry for long post please help!
Gary.

About a month back someone posted the same problem here, hard brake pedal after a rebuild or disc conversion. At least 2 replies came to say that changing your master cylinder from 1 and 1\8 to 1 inch bore will solve the problem. So I suggest you try searching this forum in the last while to read the posts, or perhaps the posters will chime in here.

Bhawk, thanks for the response I had already searched the site read a lot of posts must have missed that one, any one else? All thoughts appreciated.

Come on chevellers no help on this subject please!!! Any thoughts or past experiance???
Gary.

Unbolt the MC from the booster and measure to see what the bore size is.

Also, make sure the pedal is in the right hole on your brake pedal...should be in the lower hole.

Believe it or not, it might take two or three "new" (aka rebuilt) MC's to get a good one.

Joe

Gary.

Do remember your pedal travel decreased when you mounted your Delco M/C? The difference in travel going from a 1" to a 1-1/8" M/C bore, will be app. 20%.

I was/am chasing the same 'hard pedal' problem after a SSBC front disc conversion. My M/C is 1-1/8 and the 9" booster is in good working order. My vacuum is some 18".

Reading the part list from SSBC I found my pads are ranked 1 above street (order : street, 'my semi metalic', strip). So I swapped my pads for a more common type and that improved the pedal feel. I think the semi metalic pads need more pressure and a higher temperature to really start working. I am a moderate driver though.


Rob

Rob thanks again for your thoughts are you saying that my semi metalic pads are causing my hard pedal? Did you go to organics? Did it give softer pedal and better feel?????
Gary.

Gary.

To answer your question first: I don't really know the type of the pads, but they were not that expensive. So, I think they have a more normal type of lining. And yes, my pedal feel has improved. This is a very cheap and quick (test) striving for improvement.

Furthermore, from what I have read the past months:

1) Year One sells M/C. When you look at their offer, you will notice that one meant for semi metallic pads has a smaller bore!

2) Every caliper/pad/rotor combination needs a certain amount of brake fluid pressure. Assuming the booster does its job, like I expect yours does as does mine I am sure, the pressure is determined by the force on the pedal (rod in lower hole for a power setup, no discussion about that), the bore diameter and the routing to the caliper (valve, brake line and hose).

It could be the (combination) valve is at fault. Mine (GM# 1257177, ACDelco# 172-1350) was new and is listed in the Delco site for an enormous amount of 71-75 make/models. I really do not doubt my valve.

My (front) brake lines are new replacements for the ones that were in the car for the drum setup. I have never read the lines should be enlarged when going to (front) discs. On top of that the fittings (both on the valve side as well as on the caliper side) do not allow a larger diameter.

The bore size has a direct influence on the pressure to the calipers. I think I read in your other post that your pedal travel is very short now which is consistent with a large bore diameter and more pedal force.
Like I mentioned in another post, when swapping a M/C, the relation between 'old' pedal travel L1 and old bore size d1 and the 'new' versions L2 and d2 is : L2 = (d1/d2)*(d1/d2)*L1
Fill in your numbers and see the difference. From a page from mpbrakes I quote "going from 1-1/8 to a 1 bore the pressure will increase 25%).

Swapping my pads helped, if I find one I will buy a M/C with a smaller bore (1"), but I am not in a hurry any more.

Hope this helps.

Rob

Rob, I really appreciate your time! I have just ordered a new booster, From Steve Gregorie and a new Wagner master cylinder 1 1/8 :confused: and a hand pump style pressure bleeder. I should have those on friday. OPG is takeing back my M/C Booster combo with full credit. I guess i have been told that they are built in China :mad: After instatlling and bleedig, if pedal is still hard i will switch to a OE compound brake pad :confused: Rob once again thanks much I appreciate your thoughts I will post on Saturday with results!