More braking power needed

Most fuel line isn't designed for that much vacuum, and it may be collapsing internally, causing your lack of vacuum boost with the pump.

I've replaced a few boosters and all of them required pushrod length adjustment. Not enough clearance (pushrod too long) is worse than too much clearance.

I'm also assuming you installed larger wheels and tires, which also require more braking force to lock the brakes.

Click to expand...

Is booster vacuum weak?

Click to expand...

Most fuel line isn't designed for that much vacuum, and it may be collapsing internally, causing your lack of vacuum boost with the pump.

I've replaced a few boosters and all of them required pushrod length adjustment. Not enough clearance (pushrod too long) is worse than too much clearance.

I'm also assuming you installed larger wheels and tires, which also require more braking force to lock the brakes.

Click to expand...

Hi,

 I did not replace the booster  but changed the master cylinder to a similar original one.... anyway  got later on the feeling  that maybe I should check it anyway...

Click to expand...

Here is the type of hose you need to run from the vacuum pump to the booster. This may not be the correct size

https://www.amazon.com/Gates-27233-Power-Brake-Vacuum/dp/B000CRHL62/ref=sr_1_1?keywords=power+brake+vacuum+hose&qid=1557798264&s=automotive&sr=1-1

Click to expand...

+1 on Don C's posts. Here are some easy tests. "Booster Function Test: Check pedal feel and vacuum booster function while test-driving the vehicle. With the engine off, apply the brake pedal repeatedly with medium pressure until the booster reserve is depleted. At least two brake applications should have a power-assisted feel before the pedal hardens noticeably. If the pedal feels hard immediately, or after only one brake application, it may indicate a vacuum leak or a low level of engine vacuum. Inspect the vacuum hose to the booster for kinks, cracks or other damage. Check vacuum at idle with a vacuum gauge. 

https://s3.amazonaws.com/assets.knowyourparts.com/app/uploads/2016/09/automotive-vacuum-booster.jpg[/img]To test booster function once the reserve is depleted, hold moderate pressure on the brake pedal and start the engine. If the booster is working properly, the pedal will drop slightly."

Also, 20 in/hg is a lot of manifold vacuum for a modified 429 engine to generate at idle. Is idle speed high, or timing too far advanced, do you have another vacuum gauge to compare results? I currently have a 1969 Cougar and a 1972 Mach 1 that have aggressive cams and make 11.5-12.0 in/hg and still run the booster although these numbers are near the edge of not working with the OEM single diaphragm booster. Chuck

Click to expand...

You might consider a hydroboost system running off the power steering pump. I had exactly the same problem with my 71 - 429 and that fixed it for sure. Just a thought.  Ed Raver

Click to expand...

I got this brake booster for my 71. I havnt installed it yet, but it seems to be a direct replacement.

https://leedbrakes.com/i-21155650-9-inch-power-brake-booster-black.html

Click to expand...

Something is amiss here. 20 in at idle is a lot, what cam are you running? I am only getting around 11 - 12 in at idle with a 351c and I can lock up all fours no problem with 17x9 all around and stock brakes.  I do not think your issue is a lack of vacuum.

Click to expand...

I have  71  Mach 1 with a modified 429 engine ( car originally  with 302 engine).

Has  original disc brakes front ( never checked the pads) and drums  at the rear.

I renewed the  bleeding master cylinder with the same original  type but forgot to check the piston length ( assuming it does not need  adjustment)

I tested the booster and  it works ( pedal goes down when starting the engine).

The vacuum  (with the new engine) taken from the  intake manifold straight to the booster gives  20 inHg vacuum ...but

I would feel more comfortable if I  could lock the brakes when needed ( to find more power) than it is now. Actually the car withe the old  engine  could lock the brakes when the brake booster was connected to the intake manifold...

What could I do next ?

Before measuring the  vacuum from the intake I  tought that the  new cam  is  causing  too little  vacuum for sure but apparently  as I know now it should be just fine ( altough on the minimum side = 20 inHG).

Therefore I decided to buy a  Hella  UP 30  vacuum pump . When measured  - straight from the pump intake - it  gives 24 inHG vacuum. I did not yet measure it  through the whole  pipeline  at the booster with check valves and other stuff attached - but will do later to see if there is some loss of vacuum on the way.  Anyway when the line was ready installed  and I tried the car  - braking was disappointing  even worse  than with the  intake manifold

booster installation.....   the  pedal was stone hard ......so I am  a bit confused.

The line  is mostly 3/8 strengthened fuel rubber hose

MORE TESTING !!!!!!! 14.5.

I connected the  Hella Vacuum Pump directly to the booster and  YES - I found more brakes but could not lock them up yet - the  vacuum is 24inHg !!!!

I also measured the vacuum with the pressure switch from Summit ( and it seems to be used in several kits as well)  and it cut off at 14 inHg !!!! What the Hec !!

So the vacuum pump cannot deliver full power as it is cut off so early.... any alternative to that switch - adjustable perhaps  for higher  vacuums?

My friend  asked if the brake master cylinder has the same bore as the orignal had - I told him I do not know as I  bought  a similar  original cast iron  one for  71 Mach 1.... Could the reason be there and the  brake pads at front are old  but bearly used ( original type ?)

Click to expand...