Brakes....

[GaryAshyLarryBird]

I got this 76 Datsun 620 pickup a couple days ago. If I use the brakes It dies. Could this be the brake booster? Also, the brakes are really hard to push. I'm just curious to why it's dieing on me everything I touch the brakes.

[datzenmike]

Pull the brake booster hose off the intake and seal with duct tape, temporarily.

Start engine and test the brakes.

Does that fix the stalling?

 

 

Very hard pedal and engine stalling is likely a vacuum leak in the diaphragm.

[sebpv]

Mike can you explain the concept please. Just tryna learn something new here...

[sebpv]

if im correct, the brake booster sucks in air as i press the pedal. If theres a rip on the diaphragm the pedal moving the diaphragm would only cause movement of inside air but how does the leak relate to the stalling?

[datzenmike]

In line to the booster from the intake, is a one way valve. It allow the engine to draw (suck) air out of the booster, but not go the other way. This is so that should the engine stall while driving and the intake vacuum disappears, there is still vacuum stored in the booster. Now you still have enough vacuum assist to apply the brakes normally to get stopped. It would be best in this case to not pump the brakes and use it all up.

 

The brake booster is a large can with a diaphragm across the middle like a drum skin. When the engine is running It has vacuum on both sides so it's neutral. The brake pedal is connected (and sealed to keep the vacuum in) to the diaphragm. The other side of the diaphragm has a push rod into the master cylinder. If you push on the brake without the engine running, and no vacuum, it transmits pressure directly through the booster and works the master cylinder. With vacuum present as the brake pedal is depressed it opens a leak on the driver's side of the diaphragm. Normal air pressure enters and will push on the diaphragm and it in turn pushes the master cylinder push rod. So what happens is that with a small amount of effort you can leak in and apply air pressure to assist your braking effort. 

 

Lets say the diaphragm is 6" across, that's over 28 square inches of area. Now just for argument sake, say we have a perfect vacuum and we allow in just 5 PSI of the 14.7 PSI available air pressure at sea level. That's over 141 pounds of assist (on the master cylinder) on top of what your foot is pushing.

 

Naturally the assist in usually much less than that and your foot constantly adds and subtracts pressure as you brake to stabilize.

 

Generally the '78 620 braking system without vacuum will produce about 900 PSI of brake fluid pressure for 132 pounds of pedal pressure.

 

With vacuum assist working a 132 pound pedal pressure produces over 1,400 PSI. That's well over a 50% increase in assist.

 

 

OK you let off the brake the 'leak' seals and the intake very quickly removes the 'pressure' and vacuum is restored to both sides of the diaphragm.

 

When you step on the brake and air is allowed in, you may hear a very soft hiss. There is a felt 'muffler' to silence the hiss, otherwise it's loud enough to notice. Try it with the engine off. There should be some vacuum still present. If you listen you might hear a soft sigh sound.

[sebpv]

my truck is in pieces but i do hear it on my f250. I actually ripped the pedal boot but read that its not a problem? Thanks for the info. Very detailed!

[spoonman]

Bleed them brakes lol.

[datzenmike]

If the pedal is rock hard when stopping, it isn't trapped air. Air is compressible and the pedal will either go to the floor or is spongy.  Sounds like lack of vacuum assist. This makes a huge difference.

[G-Duax]

I have a leaking assist diaphragm.

A constant leak will cause the engine to run lean, and RPM to drop.

If it's a large leak, it will die.

 

Tried to super glue the small tear, then cover it with a liquid rubber compound, and it fixed the problem for about 2 weeks, but now it's coming back again. Doesn't die, but idles way down until I let off the brakes.

 

Sure wish I could find a replacement diaphragm, but no luck yet.

Stuck with the questionable cardone junk for now.

 

Searching such things as power assist clutch masters now.

The 620 has a couple of different assist diameters, depending on year.

I think my '73 is about 4.5". but it's been 6 months since I had it apart.

[TDCTDI]

I have a leaking assist diaphragm.

A constant leak will cause the engine to run lean, and RPM to drop.

If it's a large leak, it will die.

 

Tried to super glue the small tear, then cover it with a liquid rubber compound, and it fixed the problem for about 2 weeks, but now it's coming back again. Doesn't die, but idles way down until I let off the brakes.

 

Sure wish I could find a replacement diaphragm, but no luck yet.

Stuck with the questionable cardone junk for now.

 

Searching such things as power assist clutch masters now.

The 620 has a couple of different assist diameters, depending on year.

I think my '73 is about 4.5". but it's been 6 months since I had it apart.

I've not had one apart, is it a flat diaphragm or a moulded shape?

 

I have made my own "silicon" hoses & repair parts by taking a piece of cloth & working RTV into it with a putty knife & wrapping the hose with it or or a mold of the part itself.

[G-Duax]

They are molded, like a tube, that rolls into it's self.

Kind of like rolling up a shirt sleeve.

 

I may try a hot vulcanizing type patch next time, if all the first patch junk can be scraped off.

[TDCTDI]

Or maybe a bicycle tube patch?

[G-Duax]

Yeah, would probably be better than the quickie fix I tried.

Sometimes my Wisconsin farm back ground kicks in.........

[Stoffregen Motorsports]

I just checked Rock Auto and you can get a reman booster for less than $100. I wouldn't mess with trying to fix one for that price.

[TDCTDI]

I just checked Rock Auto and you can get a reman booster for less than $100. I wouldn't mess with trying to fix one for that price.

X2