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Hey guys I'm new on here figured I'd make a post. I have a 53 Chevy 1.5 ton flatbed I've been working on. Original drivetrain with some minor upgrades....working on my brakes now mounting an under the floor vaccum booster and dual master cylinder. Hoping to find some answers on what could be wrong or what I should do differently. I'll post pictures but what I did was make a new bracket and rod and mounted a vaccum booster for a 75 c30 Chevy mounted to a late 70s p30 master with 1.25 bore for power drum brakes. I then added residual valves in line and used the original vaccum line that ran to the old hydrovac system to the new vaccum booster. But only problem is that I cant seem to get brakes!!! Its like no pedal and i really dont understand what could be the trouble. I've seen many other have had success but mine has the two wheel cylinders in the rear drums does that make a difference?? Any advice would help!!

Some none vehicle specific suggestions:

The master cylinder might need to be bled before mounting to the truck, get the air out on the bench, then install and bleed the system normally. I do that with any master cylinder work including hydrovac units, pull the master off it and bleed it on the bench. It is much easier to assure that all the air is removed this way.

The push rod to the master might be too long, it needs to have enough slack that the small feed hole at the bottom of the fluid reservoir is uncovered when you release the pedal, allowing fluid into the actual cylinder and releasing pressure built up in the brake lines. The hole is covered and pressure is built up when the pedal is depressed.

There may be one or two bleeders on the hydrovac, both need to be clear of air.

There is a specific procedure for bleeding the dual wheel cylinders on the rear. Do a search using the search function in the bar just above your post.

Thanks guys yeah I'll look into that! I was hoping someone that has done this conversion would be able to see it and help! Lol

I'm at a loss, I bled the dual master cylinder on the vice it's for a late 70s p30 "power drum"...I have residual valves close to the master cylinder..my push rod pushes the booster in only about a 1 to 1 1/4. And I made the rod with the pedal all the way out and the booster all the way back to the normal position. Anyone who has done an under the frame mount please help.

Regarding the push rod, to be sure there is enough travel in retract direction of the master piston, there should be a squirt of fluid out of the bleed hole in the reservoir when the pedal is pushed. It often results in a fountain shooting out of the master making a mess if you push the pedal quickly. The cover needs to be off so you can see it. Push gently.

Do you have adjustment in the rod? Adjust it so there is a little free play in the rod when the pedal is all the way up. The piston in the master will be against its retaining ring at the front of the master. Bleed things, then adjust the play just out. If you do not have adjustment in the rod getting the right length can be near impossible. It needs to be correct within perhaps .030" or less to work correctly.

A master cylinder from a drum brake system should have the residual valves in the outlet ports already, there should be no need for external valves. I don't know what happens if there are two valves in line but I would make sure that there was only one.

Very good information sir, yes in fact I do have the little squirt come out of the bleed hole!! I'm happy that's normal because I was wondering! The rod yes I can adjust it but I'm confident it has to be dang close but I will recheck everything. And the reason I added the residual valves is because yes for drum brakes especially being under the floor mount they say to have them but being that my master is for a 75-81 p30 Chevy I ordered two because **correct me if I'm wrong** after 1974 most master didnt have them because a change in the wheel cylinder cups or something like that. I also read a time or two that extra ones in series wouldn't effect anything but like I said I'm open to all inputs. My next step is pull my hubs off and look at wheel cylinders to see what's going on in there. Other than that I'm hurting for answers I literally have done almost every option I can think of ha.

It sounds like you have the rod right and the master should be free of air. You have an "under floor vacuum booster" you say, I assume that is a vacuum unit attached directly to the master. If it is separate from the master, as in hydro-vac, it needs to be bled too. You probably know that.

I don't know what the brake arrangement on a 75-81 P30 is but you said the master was for drums in your first post. That would tell me that it had residual valves in the master. If it was for disc front, drum rear, it should have a valve in the port for the original rear drums. That would normally be the port to the front as mounted in the original vehicle (I am not familiar with the P30 arrangement). If you have information that adding more residual valves does not cause difficulties then I am out of suggestions. Someone else will know more and I am pretty sure they will be along to help.

Yea friend you have been very helpful. It's a actual vaccum booster like a mid 70s firewall mount style only I made a bracket and mounted under the cab. My hydro-vac kept giving me problems and I've read different posts of guys doing it and I'll tell you I never thought it was going to give me such a headache ha ha. I ordered the master new from an online supplier I believe parts geek. It has a 1.25" bore and was advertised for power drum brakes so I thought thatd be my best bet...one resivore is slightly bigger but not that much so I dont believe that would be an issue. I adjusted all my wheels to have just a slight drag, the only concern I have is maybe I'll have trouble bleeding them because its an under floor mount and the mc is not much higher than the wheel cylinders....maybe I need to get it pressure bled?