I installed a new MC. with only a dimple end not a 1.6 hole not realising it should have been, as I was tightening the screws there was a fairly loud crunch noise and after. changing the piston and bleeding the brakes I have no power brake operation obvious I have broken something, The brakes work but no power assist. What have I broken I am out of my depth now and need advice.

 

If I had to guess I'd say the rod pressed back into the booster and pushed on the piston/diaphragm inside the booster far enough that it broke/cracked so it can't build pressure (vacuum)
M

 

Would you like me to change your thread title to something related to the subject of your post, so people will have a reason to read it?

 

If that is the case... OH, NO!!!

 

Since he said the brakes work without the power assist... At least taking those 4 bolts that hold the booster on are easy to get at.....
M

 

Booster help

 

It would be a really good idea to do a better job of communicating. What kind of booster help? How to remove? Where to get rebuilt? Where to buy a new one? What length rod to m cylinder? Etc. We are willing to help, but we are not mind readers. We don’t even know what year we are talking about.

 

I believe he was just responding to vettebuyer6369 offer to re-title the original thread. His first post does explain what he was doing, what happened and the resulting issue.
Would always be nice to know the year of course

M

 

1975 C3 5.7. The site is very useful in gaining information and I appreciate it, unfortunately I trusted the supplier of the MC as they advertised it as suitable. The reason that I had to replace the the MC was because the previous owner had gunk in the reservoir and because I had the car on stands while I made and installed SS side pipes with Aluminium shields which I made myself, it took a while therefore the caliper seals leaked, so I decided to replace the system. I turned up sleeves to help install the pistons & seals without damage I haven't done to bad for 78 next year. I would like to post a photo but not flash on Computers

 

Very simple here:

New (2020) Picture Posting Instructions - CorvetteForum - Chevrolet Corvette Forum Discussion

 

Before problems

After making Side-Pipes

 

very nice
M

 

Back to you problem, I'm still thinking that since you say the brakes still work it must be the booster.
You can test a booster without taking it off (your 77 year old back ain't gonna like getting at the bolts for the booster, trust me...)

Here's some test I stole from the net

3 Methods for Testing Power Brake Booster Failure

Method 1: Testing a power brake booster is a fairly simple process. If you suspect the power brake booster is the root source of your brake system failure, complete the following three steps:

1. With your engine turned off, pump the brake pedal several times. This will ensure that there is no vacuum remaining inside the brake booster.
   
2. Press the brake pedal down hard one final time and leave your foot on the brake pedal as you start the engine. Do not release your foot from the brake pedal during this process.
   
3. If the brake booster is working correctly, you’ll feel a slight give in the pedal as the engine turns over. This is because the vacuum in the engine is supplying pressure to the brake booster.
   

Method 2:If you complete this step and the brake pedal does not move, it is an indicator that the power brake booster is not receiving vacuum pressure. This is when you should attempt a secondary power brake booster test.

1. Let the engine run for a few minutes.
   
2. Turn the engine off, then repeatedly press the brake pedal slowly. When you pump it the first time the pedal should be very ‘low’— meaning not much pressure resistance. As you pump the pedal, the pressure should become firmer, which will indicate that the brake booster is not leaking.
   

Method 3:If each of these tests passes, there are two more components you can check:

1. Inspect the Booster Check Valve: The check valve is located on the power brake booster itself. To locate it, consult your vehicle repair manual. You’ll want to disconnect the vacuum hose as it connects to the engine’s intake manifold. Make sure to disconnect it on the manifold and not the brake booster. If it’s working correctly, air should not flow with pressure applied. If air flows in both directions or you can’t blow air through, the valve is damaged, and the power brake booster will require replacement.
   
2. Verify Vacuum: In order for a power brake booster to work, a minimum amount of pressure is required. You can test the vacuum and verify that there is a minimum of at least 18 inches of vacuum pressure and no vacuum leaks.
   

M