[aosborn]

Verify the bore of the master cylinder. My guess is that it is 1 1/8". Install a smaller bore master cylinder to increase line pressure. If all else is good, that will solve your problem. Dropping down 1/16" in bore size makes a very noticeable change in pedal feel. If it is as hard of pedal as you describe, you might consider going to a 1" bore diameter. You will increase pedal travel and increase line pressure.

Regards,

Andy

 

[aosborn]

To measure the bore diameter, remove the master cylinder from the booster (often you can do this without removing the brake lines) and with a caliper measure across the hole that the master cylinder piston rides in. That is the bore of the master cylinder.

For power brake systems on hot rods, 99% of the master cylinders out there are either 1", 1 1/16" or 1 1/8". GM did make some step-bore master cylinders, but they are uncommon and not typically used on a hot rod.

Many of the commonly used master cylinders that come with the aftermarket kits have different bore diameters available so you won't have to redo your brake lines to swap it out.

Keep us posted on how you are doing.

 

[aosborn]

If you could post a photo of the master cylinder I would like to see the one you are using.

Measure the bore again with a tape measure and see if it is still just over an inch.

Have you checked the adjustment of the push-rod between the booster and master cylinder for proper adjustment? There should be just enough clearance so the pushrod doesn't push the master cylinder at all when it is bolted to the booster.

 

[aosborn]

Hers's the deal...Typically a kit purchased for a disc/drum combo will come with a 1 1/8" master cylinder as you don't need as much line pressure with that combo.

If the kit came with a combination valve it will be incorrect with a disc/disc system because of the 10lb residual pressure valve going to the rear brakes and the metering portion will be wrong. It sounds like you had a stand-alone residual pressure valve, and in that case you should have switched the 10lb for a 2lb. The 2lb check valve keeps the pads from backing away from the rotor when you are not pressing the brake pedal. Also make sure they are installed in the proper direction.

With your booster, a 1" bore master cylinder with a 2lb check valve in the line to the front brakes and another in the line to the rear brakes along with an adjustable proportioning valve, (or the proper combination valve) will give you good brake action. I will assume that you haven' modified the brake pedal assembly as it came from the manufacturer.

Does the master cylinder you are using have a deep (about 1") depression in the rear of the piston, or a shallow one (about 1/4")? In most cases you need the shallow one with a power brake booster. Again, a photo showing the unit and the piston would be most helpful.

Lastly, what type of calipers are you using on the rear?

If we beat on this long enough it will eventually give in and work for you!

Andy

 

[aosborn]

That is a very common master cylinder. They came installed on 1982 Corvettes if I remember correctly. They are available in 1 1/8" and 1" bores, but I don't think they came in 1 1/16".

With the master cylinder bolted up to the booster, how far can you pust the rod on the pedal side of the booster before you feel it compressing the master cylinder piston?

 

[aosborn]

this link is for the same master cylinder Classic Car and Truck Brake, Steering and Suspension Parts

It may or may not say disc disc or disc drum but when I bought it I told them disc drum and all they said was I would either use it as is for disc disc or add the inline valve for disc drum.

As for the booster, I have the adjuster nut on the booster turned all the way in and it feels like it is just barely touching the master when I bolt up the master to it. The pushrod between the master and booster is non adjustable.

When you say you have the adjuster nut on the booster turned all the way in, which nut are you talking about? Is it the pushrod that connects to the pedal?

 

[aosborn]

Interesting...

What it looks like you have is a manual brake master cylinder with an insert installed to take up the space to the booster pushrod. Not a problem, but I haven't seen that before.

You also have a combination valve, and if I understand you correctly that is the same one that you used when it was disc/drum correct? If so, that is the wrong unit for disc/disc. Did you remove the drum brake check valve out of the combination valve, out of the master cylinder (some masters have them built in) or was it an inline valve?

What I would do is this. Disconnect your brake pedal from the booster so it isn't pushing against it at all. Put that spacer into the master cylinder, lengthen the pushrod coming out of the booster quite a bit. Then, slide the master onto the booster and you should feel the piston hit the pushrod. If so, shorten the booster pushrod and retest until you can just slide the master onto the booster without feeling resistance, then shorten it about 1/2 turn more. That way you can be sure the booster isn't preloading the master, and that there isn't too much clearace. Bolt on the master cylinder to the booster. Then, adjust the pedal pushrod until you get the pedal height you want and tighten the jam nut.

Now is where it gets interesting. If you like, bleed and test the system and see if it is any better. If not, remove the combination valve and set it aside. Install a 2lb check valve, one in the front lines near the master cylinder, and one in the rear line along with an adjustable proportioning valve in the rear line, bleed and retest. I think then things will be much better.

I still would like to know what kind of calipers you have on the rear axle.

 

[aosborn]

That "master cylinder pushrod" is just a filler that company makes so you can use a manual master cylinder with a power brake booster. Since you can still adjust the pushrod in the booster, my technique for adjusting the clearance between the master and the booster is still valid.

Since the combination valve is for a disc/disc setup, you should be okay there. One other little quick check is to make sure there is no residual pressure check valve in the master cylinder itself. Take a small allen wrench or drill bit and put it into the fittings on the master where the brake lines attach. If there is a check valve, the tool will only go in about 1/8" past the seat that the brake line seals on. If there is no check valve, it will go in about 1/2" or a little more. You are probably okay there, it is just good to check.

Make sure that if there is a way to adjust the parking brake on those rear calipers that you adjust them up snug.

 

[aosborn]

You only need to check the ports that you are using. Having that check valve in the master when you don't need it won't make for hard braking, it will just make the pads drag on the rotors. It is good to check for it. If you find one, there is a way to take them out. Just let me know and I can talk you through that.

The lack of the check valve, and large reservoirs are the charactersitics of a disc/disc master cylinder, so if there are no check valves that master cylinder is fine to use.

It is possible that if the emergency brake is not adjusted properly it could reduce braking performance. I would do a GOOGLE search for those calipers and see what others have to say. The calipers on the 9" Ford rear axles out of the Lincoln Versailles are famous for that problem. Unless the parking brake is adjusted up tight, you will never have good rear brakes.

Keep us posted on your progress.

P.S. That combination valve looks pretty low to the ground. It isn't gonna drag on speed bumps or anything is it?

 

[aosborn]

Very good. Just thought I would ask.

Once you have the booster and master cyl set up properly, we can get into the bleeding process (that is brake bleeding, not human bleeding!).

Keep us posted...

 

[aosborn]

What I am really interested in once it is all put together is how much fluid volume you are getting at different points in the system. Do you get a real strong shot of fluid at all the calipers when you are bleeding them? Sometimes the front will have strong volume and the rears, not so much. There are different reasons for this that we can talk about if necessary.

Good luck!

 

[aosborn]

Glad we are making progress...

Does the pedal "pump up" when you are pumping the pedal, or does it stay at the same height every time you push it?

If you hold firm pressure on the pedal does it sink to toward the floor?

How high above the floor is the pedal when you are pushing on it hard?

If it stays the same and doesn't sink down, then try this...

Go into a gravel parking lot and drive about 15 mph and hit the brakes hard. If any wheels lock up, you will be able to see by the ruts in the gravel which wheel or pair of wheels it was.

Let us know what you find.

 

[aosborn]

The stumble is more than likely a carburation issue and not due to vacuum.

It sounds like you are right on with the master/booster adjustment. Leave it as-is.

Cheap pads usually equal soft pads. Soft pads usually stop better cold than high performance pads. Don't get me wrong, there are some cheap pads out there that don't stop for a damn.

Lets keep working with what you have before you go and spend any more money.

 

[aosborn]

Now try this. With the engine off, press on the brake pedal and hold it down reasonably hard. Then, without lifting your foot, start the engine and tell me what happens.

 

[aosborn]

What type of hose or line do you have connecting the booster to the engine?

 

[aosborn]

Auto parts stores sell a special hose for brake boosters. It is described as 11/32" vacuum brake hose. It has a thicker hose wall and won't collapse internally, and being a slightly smaller diameter you don't typically need hose clamps if using it on a hose nipple fitting (though you can clamp it if you like).

For a hose run as long as yours (booster under the floor) I would recommend 3/8" hard line from the intake down along the transmission to near the booster, then a piece of vacuum brake hose from the hard line to the check valve on the booster.

At very least replace all the hose you have with the special hose.

Once all that is good, we can look at brake pressure at the wheels.

 

[aosborn]

The next question is how much flow of brake fluid you get from each caliper when you are bleeding the brakes. Do you get about the same volume of fluid at each bleeder screw when you are bleeding them?

If all that looks good, jack up the truck and get all four wheels off the ground. With the engine off, have a helper apply the brakes until you can't turn the front tires by hand and then have your helper hold the pedal in that position. Then go an try to turn the rears and see what happens. If the rear wheels will still turn, have your helper press harder to see if they can lock them up. Sometimes I will take off the tires and use a bar to turn the wheels. If the calipers are way out of adjustment it will show up with this simple test.

If the rear tires lock up first we can address that issue too.

Did you try locking up the brakes in gravel?

Let me know what you find...

 

[aosborn]

I have seen this many times. If the fluid at the front calipers is running clear you are probably good there. As for the rear not flowing as much fluid, there still might be air trapped in the system, or you haven't moved the rear caliper pistons out enough to allow much fluid in the calipers.

With a helper pumping the pedal, start bleeding the rears right at the outlet of the master cylinder, then at the outlet of the combination valve, and then back at the wheels. See if that helps.

Also, activate the parking brake levers on the rear calipers and see if the parking brake mechanism moves the pads.

Lastly, test the brakes as I have described in an earlier post while the truck is jacked up off the ground.

Andy

 

[aosborn]

One line to the rear split at the rear end is fine.

Have you verified there's no rear brake proportioning valve in the MC? Is there an added-on adjustable prop valve?

The valve in the master cylinder would be a residual pressure check valve which does not provide the same function as a proportioning valve.

The residual pressure check valve in a drum brake application keeps the return springs on the brake shoes from retracting the shoes away from the drum, and keeps the seal on the wheel cylinder pressed out against the bore.

As to the re-bleeding of the brakes, what I am attempting to determine is whether the amount of fluid coming out at the various points is constant. Example, good flow right at the MC, good flow at the output of the combination valve, poor at the calipers. It can help to determine problems.

Also, it isn't surprising to find air trapped in a brake system that has been thoroughly bled. It won't hurt to go through it again starting at the MC. If it is indeed thoroughly bled it won't take but a couple of pumps at each point to confirm it. Also, make sure you have a helper working the brake pedal and don't just depend on a vacuum bleeder or pressure bleeder alone.

Another thing to keep in mind, is that until your brakes are bedded in, you won't get maximum stopping performance. Also, unless you are moving pretty fast you may not get lock up on dry pavement when you have wide grippy tires. That is why trying them on gravel (if you can) or wet pavement is a good indicator of what is going on.

 

[aosborn]

The following explains what I'm getting at: http://www.classicperform.com/Instructions/PDF/Prop_Valve_Mod_Instructions.pdf. If this hasn't been done, the pressure to the rears will be insufficient. A stock-type combo valve is modified in a similar manner.

So what they are telling you with these instructions is that you don't need a proportioning valve with disc/disc brakes? They make no mention of a residual pressure check valve which is in the area of 10lbs for drums and 2 lbs for discs. I would suspect that the 10lb valve is in the outlet fitting that is screwed into the combination valve. If you have a 10lb check valve on disc brakes, they will drag heavily. This modification does not make sense, and just reinforces my belief that stand alone check valves appropiate for the braking system, and an adjustable proportioning valve makes the most sense with a hot rod or custom car. One size fits all combination valves are a crap shoot if they will work properly.