I've recently changed the front brakes on my 68 Superbee from the factory drums to a single piston disc conversion.
I have continued to use the std master cylinder and booster.
When travelling at high speed for any length of time I get a vibration thru the steering wheel.If I apply the brakes the vibration stops and then returns a few moments later.
If I pull over to the side of the road the wheel centres feel warm(not too hot to touch)
I'm thinking that the vacuum is causing the brakes to apply very slightly when i'm driving.
With the booster disconnected there is no problem.
So.Should I have changed the booster/master cylinder when I did the conversion? If so what should I have changed to ?
Thank You gentlemen

Suggest you re-check wheel bearing adjustment first. If they are a little loose, that could cause the shake.

If the brakes were dragging, both the hub and wheel would get pretty hot.

While it is not your immediate problem, running discs with a drum master is a bit risky. The master does not have have the fluid volume to keep the pistons full as pads wear. Suggest upgrade to disc/drum master.

Yeah I would be looking at bearings first and the rotor assembly second.

Did you add a proportional valve?? A lot of kits come with a simple in line valve to even out the pressure

We bought a 69 Charger race car with a factory drum brake master and the previous owner changed to 70 disc up front. The drum brake masters have a check valve that keeps the fuild pressure from bleeding back down and keeps the shoes close to the drums. If it didn't, you would have a soft pedal. As the drums get hot they expand away from the shoes. Disc brake masters do not have this check valve. They must bleed off the pressure when you let up in the brake. If it does not bleed off, the rotor gets warm, expands into the pads, and slightly applies the brakes without you pushing the pedal down. We changed to a modern aluminum master for disc and picked up a tenth in et and 2 mph in the quarter mile! Just change to a disc brake master cly. Dr diff sells the one you need.

That's true but I doubt it's causing that amount of vibration

Thank you everybody.
I'll check all the things you have all suggested.
I guess I'm likely to need some new parts here.
Thanks again
Simon

Residual pressure valve in a drum brake master cylinder.

You absolutely need to change to a master cyl designed for front Disk Brakes / rear Drum Brakes. Dr. Diff sells a good one with an adapter to fit the original four hole design to the two mounting hole Aluminum master cyl and he has available an adjustable pushrod to with it. I bought the kit for my Ramcharger from him...one of the good guys!

with the booster disconnected there is no problem. that is what Sherlock Holmes (from your neck of the woods) would call a (significant) clue. how are you checking it with the booster disconnected? I would as said check the brake assys for anything loose/rubbing & first I would check the booster pushrod round nub clearance into the MC rear piston "thimble" bottom. (want ~.020"). holler back when you can.

Hi Robert,
With the booster disconnected I don't get the vibration. I do know what the booster pushrod nub is and I understand that I need clearance but I've no idea how to check it.
The engine makes a lot of vacuum and even the lightest pressure on the brake with the booster connected gives some sharp braking. One theory that has been given to me is that the actual weight of the pedal is pressing on the brake slightly and this combined with the sharpness of the vacuum is applying the brake slightly.What do you think?
I've relized that changing the master cylinder is a good idea and I will try to find a stock looking replacement,however should I replace the booster also?The one which is on is the one that came with the car and is of a split two piece design with a clamping ring and the diaphragm in between.
Sorry it's taken a little while to gwt back to you.I've been busy getting the car ready for the Classic and Sportscar Show at the NEC

No rush, here we have nothing but time. lets see if we can fix what we have right now. You would measure the depth of the rear piston "thimble" recess in the back of the MC with a spare engine pushrod out to the flat MC flange then measure the distance the rounded booster rod threaded nub is protruding out from the booster flat flange & you want that dimention to be ~.020" less than the thimble depth. A bit more than 020 is OK but not less than 020

Good input, I would only add that checking the center wheel temp may give a good indication if the brakes are indeed dragging

excellent point on the RPV potential. take the line off from the large MC (disc half) port next to the firewall & stick a drill bit into the brass cone & you will feel it if it has one, it is just a piece of neoprene on top of a spring, a crude 1 way check valve but it is enough to cause havoc on a disc system. EDIT correction: stick the bit into the smaller drum port as that is the one that might have it, the disc half would not have one

Thank you both gentlemen. I'll get on with it

You can thread a screw into the seat of the residual valve and pull it out with Vise Grips, or whatever. Take out the valve and spring, and reinsert the seat after removing any burs from the screw. The brake line doesn't touch that part.

The drum booster will not function properly, you need the correct booster. Give Dr. Diff a call, he has the correct repro. Been there, done that.