[pat55ht]

I have just installed a new 8 inch power booster and new master cylinder on my 55 Chevy, but there doesn't seem to be any improvement over the non-power. There appears to be good vacuum but i haven't put a guage on it. the engine is a stock 350 with Edelbrock carb and manifold.
The master cylinder is 1 inch bore, which i was told would be better than 1 1/8 to use with the power brakes. the lines going to the porportioning valve are 3/16. the vendor assured me this was ok. i have front disk brakes with drum on the rear.

does it matter if the vacuum line fitting is at the top or bottom?

i would appreciate any advice.

 

[Ace57]

Could be some air in the brake lines....or the new Master Cylinder. Did you bench bleed the new MC? Also, check to be sure your pedal rod, at MC and at Booster isn't pre-loading (rods too long) .....you should have 1/16' or less play down the line. Rod adjustment is critical.

There's lots of members on this forum that should be able to help you...and keep reading prior posts.......there are dozens (probably hundreds) of them on this subject.

Good luck, I'm sure you'll get it figured out.

Ace

 

[Rick_L]

A 1" master cylinder will have more line pressure or more pedal travel than a 1-1/8" master cylinder.

But that doesn't seem to be your problem.

I would be looking at vacuum issues, as well as whether the booster is even working.

There is a good vacuum booster troubleshooting writeup at mpbrakes.com.

 

[JB55]

With the engine off pump the pedal a few times and then hold the pedal down. Then start the engine, the pedal should drop a little. This should tell you if it is a vacuum/ booster promblem.

 

[105thking]

Bleeding

By converting to front disc brakes, I don't believe you would notice a great deal of difference of "stopping distance" between drum and disc, since drum brakes actually have more surface and stop better. The idea behind disc brake installs is that they are simple, and with power assist they take less pedal pressure to stop. If you purchased an aftermarket kit I'm sure all the componets are matched. One of the most "impotant" steps to disc brake installation is bleeding. The bleeder screw on the caliper "must" point at 12 o'clock so all the trapped air can be released. No other position of the bleeder screw will assure you that all the air has escaped.

 

[chevman57]

The 8" booster made a world of difference on mine over a 7" one i had on my 57. Doesn`t make any difference if the vacuum port is placed at the bottom or top.
Terry

 

[pat55ht]

power brake question

Thanks to everyone for your help.

 

[tewitt1949]

I converted to after market duel power brakes on my 56, drum all the way around and it made a big difference. Almost too senitive. Works great.

 

[MikeB]

3/16" lines are fine.

Not sure what you mean about the vacuum fitting position. ???

Is it a dual or single 8" booster?

How much vacuum does the engine make at idle? And is your booster vacuum source the large port on the rear of the carb or a manifold port? (Either is fine.)

How does the pedal feel -- hard, spongy? Does it work better of you pump it a couple times?

 

[pat55ht]

Brake booster question

it is a dual diaphragm 8" booster. Summit's generic brand.

The pedal feels hard and it doesn't feel like the power booster kicks in until the car is almost stopped.

the vacuum port is on the manifold (Edelbrock) behind the carb. 20 inches vacuum at idle.

i have replaced the master cylinder (same as the old one) because the check valve seemed to be failing intermittently.

Before that i had done the following tests. Test 3 was the only one that failed. i haven't tried it again since replacing the master cylinder.

Test 1

With the engine off, pump the brake pedal to remove any residual vacuum in the booster.
Hold pressure on the pedal while you start the engine. When the engine starts, the pedal should drop about a 1/4", this indicates that the booster is working properly.
Test 2

Run the engine a couple of minutes.
Turn the engine off and press the pedal several times slowly. The first pump should be fairly low. The second and third should become slightly firmer. This indicates an airtight booster.

Test 3

Start the engine and press the brake pedal, then stop the engine with the pedal still pressed. If the pedal does not drop after holding the pressure on the pedal for 30 seconds, the booster is airtight.

Inspect the Check Valve

Disconnect the vacuum hose where it connects to the intake manifold. Do not disconnect the vacuum line from the booster. Air should not flow when pressure is applied, but should flow when suction is applied. If air flows in both directions or there is no air flow, the valve needs to be replaced.

Thanks