Truck's got Power Brake issues (disc/drum)

[CochinoFilipino]

Her brakes don't feel right. When I apply them they work okay, but as the pedal travels, it sinks down midway and I have to apply more pressure to continue stopping.

I bled the the brakes. That didn't help at all.

I'm thinking I've got a vacuum leak somewhere.

Can I hook up a vacuum pump and gauge to the booster to test it? How much vacuum should I pull to test it? How long should it hold?

How much vacuum should I see coming from the manifold? And how do i check that? Disconnect the booster, or T' into the line?

Thanks,

[70c10]

Did you swap out the proportioning valve?

[CochinoFilipino]

I didn't do anything other that bleed it. Whether it has a proportioning valve that is correct for disc/drum, I don't know.

I bought it the way it is. It's had the funny brake feel since Day 1.

The rear was locking up before the fronts too. I haven't noticed it doing that since I bled the brakes, but I haven't driven it very much either.

[67ChevyRedneck]

It could be a couple things, if you have disk brakes the master cylinder should have two different sized reserviors the one towards the front should be bigger. If you had a drum/drum reservior then they would be equal in size. Yes, power disk brakes need to have a proportioning valve to work properly. Are you losing any fluid, or do the brakes just feel mushy? When I did my power disk brake swap on my 67 pushing the pedal down more than an inch or so would put some instant tension on the 3 point seatbelts

[CochinoFilipino]

The MC reservoirs appear to be equal in size. The lines coming out are not the same size.

There is some fluid leaking out over the top of the front reservoir.

If I need a new MC, what exactly do I need?

Does this look like the stock 4-drum distribution block?

[jbon64]

my 93 gmc had the same symptoms. soft pedal , but still stopped. did every thing you did and finally fixed it by replacing the rear wheel cylinders.

[Smallz72project]

My 72 GMC has that same MC, and appears to be the same Distribution block, but mine is non assist, and I have front disk and rear drum. Your problem isnt vacuum related, if it were, the pedal wouldnt be soft but hard. Did you bleed the MC as well. You can attempt to pin point the problem by disconnecting and isolating compontents. First try bleeding the MC by one at a time just cracking the lines going to the MC, if that doesnt work, get a MC plug, and disconnect one line and then plug the MC and try it, if it still is soft, that area weither it be the front or rear isnt the problem, do the same for the next line, if soft then its not just the the next system, go through and test it down the system. if it still doesnt work, then most likely the MC is bad. I work at les schwab and see this come in alot. Sometimes there is just an air bubble stuck in the system way up there that just wont get out, if you have access to a gravity bleeder, hook it up, if not have a local shop that has one bleed the system for you. The way it works is it attaches to the MC and applies air pressure to it, you open up the bleeders and it forces the fluid out while at the same time keeps the MC full so it constantly feeds fluid. Hope this helped ya at all.
Keep on truckin

[dyingdeadman]

just sounds like a bad mc to me.

[truckdude239]

sounds like a bad mc i had trouble with my non power5 brakes softpedel but stoped then one day it not stop anymore new mc for 15 buxs but if i was you i'd replace the p. vavle

[CochinoFilipino]

I'll be working on it this weekend. If it turns out to be the MC, what should I get as a replacement?

I don't think Disc/Drum was an option in 67.

[MisterMan]

Originally Posted by CochinoFilipino

Her brakes don't feel right. When I apply them they work okay, but as the pedal travels, it sinks down midway and I have to apply more pressure to continue stopping.

I bled the the brakes. That didn't help at all.

I'm thinking I've got a vacuum leak somewhere.

Can I hook up a vacuum pump and gauge to the booster to test it? How much vacuum should I pull to test it? How long should it hold?

How much vacuum should I see coming from the manifold? And how do i check that? Disconnect the booster, or T' into the line?

Thanks,

same here, except primary locks before every else does.... makes right turns quick!!! lol. right now, its just an eyesore in the drive way... i dont drive it....EVER!!