Are the rear brakes self-adjusting? If so, are the adjuster screws oriented properly, LH and RH sides?
The vacuum at idle is 18". The brake system is below the floor. I tried setting it up as manual brake but was unable to stop the car .It is because the pedal ratio is 4 to 1, 6 to 1 is needed for it to work properly .I cant fit a taller pedal as the pedal comes out to the right of the steering column, very close. There is just no room. I never was a fan of power brakes in a rod, I was not the original builder.
Just a thought, does the new booster have the same depth for the master cylinder rod? I know you say the clearance is correct but if it's not letting the master come back up fully (even if the pedal does), it'll never bleed up right and give a low pedal. Phil
To set the clearance of .020 between the power booster and the master cylinder I use a pin depth gauge tool from Master Power Brake, It looks like a big "H" with a sliding rod in the center that measures the mounted distances between the two parts. The master cylinder is definitely coming back fully. I just cant believe no one else has run into a problem like this.
The benefit would be increased volume and higher pedal. The down side would be increased pedal effort, due to lower pressure.
Yeah, I know. Was wondering how long it would take to call me out on it. That's why I posted pics of the two different seals. Just trying to be helpful through my keyboard. Carry on....
This may be a strange comment, if you have replaced any brake pipes (lines?) check the unions again for tightness, I came across this a couple of days ago helping a friend working on a 1970s car, the flexibles had been replaced along with a solid line, no way would the brakes bleed properly or stop. Kunifer lines were used ( copper and nickel alloy), these had been fitted a couple of days earlier. He had spent most of the previous day attempting to bleed the system. Years ago I was taught to nip up copper lines fairly tightly, then return later to check tightness or rather re tighten, this ensures proper sealing, as the flared end of the tubing crushes to provide the seal. The unions may not leak or even weep but air can be sucked back into the system making bleeding impossible. Many old school electricians and plumbers I know use this technique of tightening then re tightening fittings when working with copper. Old style head gaskets also required re torqueing after a couple of heat cycles, copper again.
All of my brake lines are steel ,no nickel / copper lines were used. But I will go over the tightness of the master cylinder lines, just to be sure they are tight. At this point anything is possible .
Talking about manual brake pedal ratio, you don't necessarily need a longer pedal inside the cab. You could raise the manual master cylinder closer to the pivot. How much would the pedal ratio increase if the master was 1/2 inch closer to pivot?
Did you accidentally cross the lines going into the master? Meaning, do you have the front brakes plumbed to the master cylinder chamber intended for the front brakes?
The closest calculation with moving the pivot hole up one half inch ,which was maximum available was 5.5 to 1, but the angle for the push rod going into the master cylinder too was severe.
No, the lines are correct, the front brake lines and rear brake lines are going to the correct outlets on the master cylinder.
You mean "Increases clamping pressure-NO" A larger bore m/c increases pedal pressure / effort required but decreases clamping pressure. Have you jacked up the vehicle and checked for bearing play. Your symptoms are very similar to pad knock-off or shoes needing adjustment. Also pull the hand brake up a bit, then give the brakes a pump! Then see if the pedal comes up. Check the rotors for run-out as well. And while you're at it, measure the knock-off with a feeler gauge [it should be 0.005" between the pad and the rotor] If there is no pad knock off and correctly adjusted shoes ,you don't need to change the pedal ratio. You could benefit from a smaller bore M/C with longer travel [deeper bore]
A real strange one this as its come about from a repair / replacement rather than an modification or build, and possibly why it remains unresolved? Joe, are you dead certain that the replacement parts are as per the originals in all key dimensions, with measurements checked? Oh, and the pedal at rest is fully retracted from the booster by the return spring, with a minute amount of play? You mentioned that trying to increase the pedal ratio by shortening the arm (I don't know the proper term for the master cylinder connection part of the overall pedal!) you couldn't get a good angle into the master. Fine. But what's the swing on that (name not known part) as if that isn't swinging properly (between say 4 and 8 o'clock), but is more like, say, the force drops away with the arm rotation. Shouldn't really be as issue though as it all worked previously!! I'm a little wary of the original setup working properly with only 4:1 pedal ratio and a firm pedal at 2 1/2" out of full travel of 7". Indeed I'd expect the revised situation of 2 1/2" pedal off the floor to be more like what I'd expect - I've not done any of the calcs to prove this though! If you're happy to swap parts I'd suggest you try and get an 8" double booster in there. The 7's don't do much but the double 8" provide reasonable assistance. The 8 isn't much bigger (obviously!). Back this up with a 1 1/8" master and you should have enough pressure without excessive pedal travel. Chris
I've used several GM masters with outlets on both sides, and simply bench bled one side and installed them without any issues. No reason both sides need to be bled.
The brake system was working fine for the last 10 years. A leaky master cylinder was the reason for replacing the master cylinder. It had also leaked into the booster so I replaced them both, I had a 7 inch single diaphram booster and the master cylinder was 1 inch. Part dimensions were exact ,only difference was this master cylinder had outlet ports on both sides. I have now replaced the booster and master cylinder twice with no happy ending. The brake pedal ratio of 4 to 1 is correct for a power booster system. I set the system up manually with no booster but was unable to stop the car . The lower tab on the brake arm where the rod that pushes into the cylinder can not be re-drilled to try to increase pedal ratio due to the angle causing a severe aligment problem between the pedal and master.Things are tight under the floor ,no room for a bigger booster.
At least you tried master with out booster. So pedal firmness is fine other than wrong pedal ratio. So then there's only one thing. Booster
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