Brake Booster hard pedal

[ipoppa33]

Hello been lurking and reading posts since I bought my 70 swb in Jan
already done a few things (hood spring).


Been trying to get a handle on what the previous owner did and didn't do... oh boy.
Not sure what the truck came with stock but it has discs in front and drums in the rear. Anyways we finally got part of the brake problem figured out- the rear cylinders were shot. Got new ones and re-bleed the lines and the truck will stop but doesn't feel like the power brakes. The booster is small, it holds vaccum. Have adjusted the rears and checked the fronts and everything seems fine, just doesn't stop great (can't lock them up and the front end doesn't nose dive like I think it should with discs).
Is this brake booster the correct one or should I get a larger one? Or is it something else?
I appreciate the help, thanks.

[geezer#99]

Could be they didn't change the pedal ratio if they swapped from drum brakes.
You need 4.5 to 1 for power. Drum is set up with 6 to 1.
Normally you drill a hole in the brake pedal arm about 1 to 1 1/2 inches lower then the existing one.
Another thing could be the pushrod isn't set right. When you push the pedal watch the rod to see if it makes immediate contact movement of the diaphram.

One more!!
When you bled the rear did you push in and hold in the reset button behind the rubber cap on the prop valve.

[RichardJ]

To check booster.
With engine OFF, push brake pedal three times and hold. Start engine and pedal SHOULD drop.

When bleeding the rear brakes, you simply bleed them.


When bleeding the front brakes the Metering valve must be opened. It takes 40-60 psi in the brake fluid to open it. If there is air in the line, you may not have sufficient pressure to open it.
There are two types of metering valves with the one in the second image requiring the pin to be pulled out while bleeding. The other type requires the pin pushed IN and held while bleeding.

It is the job of the metering valve to PREVENT nose under light braking.

Under light, moderate braking, say 200psi from the MC, the metering valve will be open and you will have 200 psi to the front brakes. At slow speeds and you are pressing very lightly on the pedal, the metering valve will close and you will have little or no pressure on the front brakes and no nose dive.

That of course, also means that if the front brakes are not bled properly, you will have little or no front brakes and no nose dive.

Between 40 and 60 psi is the threshold pressure of most metering valves, but it could be higher for some.

[ipoppa33]

Thanks for the replies. I will check the booster and pedal ratio later.
I don't have a spongy pedal so I think the brakes were bleed correctly.
Should the size of the master cylinder reservoirs be different having disc/drum setup? I will have to check again but I think they are the same size.

Thanks again for all the help.
Stan

[ipoppa33]

Started the truck and the pedal does drop.
Thanks.
Stan

[RichardJ]

The size of the reservoir(s) has nothing to do with how the brakes operate.

[Rod Run]

Originally Posted by ipoppa33

Should the size of the master cylinder reservoirs be different having disc/drum setup? I will have to check again but I think they are the same size.

Absolutely.

Disc/drum and disc/disc will have different size reservoirs

Also power or manual brakes dictate the size of the bores too

[Guy67UK]

Just subscribed to this thread as I have exactly the same issue, and its driving me up the wall. I have swapped out the rotors on the front new pads, stripped and rebuilt the calipers, replaced brake lines hard and rubber, adjusted up the rears, even swopped out the booster and master cylinder, put 7 quarts of brake fluid through it. I knew nothing of this bit on the metering block, but it would explain why when bleeding the brakes I was not getting much fluid from the fronts.

[ipoppa33]

Thanks again for the replies. Hopefully this weekend I'll get my problem figured out.
Stan

[fwayne]

Those smaller boosters aren't going to have the assistance of a larger diameter one all things being equal. But they should certainly help. I suspect you might have the wrong Mastercylinder to caliper ratio. a common size MC bore is 1" a 7/8" will give you more brake line pressure but require a bit more stroke. The calipers have a piston thats a certain diameter say 3" Its a simple physics of the ratio and that 1/8" at the MC makes a big difference.

[ipoppa33]

Wanted to give an update- Got the brakes working correctly now. Replaced rear brake lines and bought the correct Master Cylinder. I now have a master for disc/drum set up. Bleed everything and now the truck stops the way it should.

Thanks again for the help, this site is great.
Stan