1980 SC Power brakes troubleshooting

[Ron.G]

I am having an issue with my brakes. The vacuum assist is not working.
what steps would you take in troubleshooting this issue?
I replaced the master cylinder and there are no Vacuum leaks. No high Idle.

Any help would be greatly appreciated.

[Ron.G]

Is there a way to test the booster ?

[deemsriddle]

Make sure that it is plugged into the engine and that the line holds vacuum.

[john walker's workshop]

Check the vacuum hose where it goes past the transmission and around the torsion bar tube, for cracks. Repaired several over the years. You can test by clamping the hose up in the engine area and see if the idle goes down.

[Ron.G]

Great thanks guys. I pulled the check valve and clean it out with WD40. Blew some air through it
and reinstalled it and I have power breaks again, but it should be better.
I did not have a much vacuum coming from the hose going back to the engine so I will check the hose in the back.
Thanks for your help as usual. John great Idea about clamping down on the hose.

I joined this forum in 2003. You guys never disappoint!

[$yncro]

What are the symptoms of not having vacuum assist?
I just redid my brakes today ( 80 sc) since I installed
The fronts upside down.. but now the brakes are Uber
Good! Tho it's almost very little travel on the pedal.
Is that normal?
Thanks.
Ogi

[Ron.G]

Usually a hard break peddle and not a lot of stopping power.

[$yncro]

Ok.. I'll test drive tonight
Thank you.

[Locker537]

How did you conclude the vacuum assist is not working?

I had an intermittent brake issue a couple years ago.

Help diagnose a brake issue

If you want to check the booster, pump the brake pedal with engine off, then hold pedal and start engine. Pedal should fall slightly as the booster assists.

[$yncro]

Originally Posted by Locker537

How did you conclude the vacuum assist is not working?

I had an intermittent brake issue a couple years ago.

Help diagnose a brake issue

If you want to check the booster, pump the brake pedal with engine off, then hold pedal and start engine. Pedal should fall slightly as the booster assists.

Thanks for the tip! Mine works.
Cheers.

[Ron.G]

Back to square one again. I checked the hose coming from the air box to the torsion tube area and everything is in good shape. My Idle is set at 950 RPMS as normal. When I pulled the hose off the airbox and hold my finger over the airbox port, it holds at 950 so I know I don't have a vacuum leak.
After running the engine and investigating up front at the booster, I pulled the check valve out of the booster and there was vacuum being held at the booster. Sound of rushing air when it came out.
I am not sure where to look next.

[Locker537]

What makes you think the booster is not working?

[Ron.G]

I have a hard peddle so you really need to apply pressure to stop quickly. They feel like manual breaks but it bit harder, if that makes any sense.
When I hold my foot on the break and start the car, the peddle dose not sink at all.

I could trying bleeding the breaks and see if that helps...

[blur911]

Make sure your pads aren't worn down enough that they are being held apart by the anti-rattle spring plates, that can lead to all sorts of weird symptoms feeling like master cylinder problems. BTDT
Other than that, it sounds like perhaps a leaking diaphragm in the booster

[EC900]

For what it’s worth, I found a couple cracks in the booster vac hose that is by the pedal cluster (leads into tunnel). It made a discernible hissing sound when brake pedal was pressed. I don’t recall if the pedal was anymore difficult to press, but once I replaced the 12” section of hose it had a slight affect on the idle speed.
I’ll agree too with Blur911 that after 40 yrs the aged booster diaphragm could be the culprit.

[john walker's workshop]

Could it be frozen caliper pistons? Or hard race pads?