What would cause intermittent lack of brake boost?

[RenaissanceMan]

Symptoms: Braking requires excessive pedal pressure to slow the car effectively. The one-way valve at the booster was cleaned and verified to be working. Holding a foot on the brake while starting the car results in the pedal dropping as would be expected for a correctly operating booster. But here is the strange part. Occasionally the brakes feel fine after the car sits for a while...? In my experience, a failing brake booster is not intermittent, but I was wondering if any of you had experienced this before. Calipers have been rebuilt and the brakes bleed with good fluid flow and no issues... Hoping to get this mystery figured out!

[rridge]

Failing master cylinder, failing booster bladder, worn out check valve, incorrect type or too old vacuum hose. Conventional heater hose substituted for vacuum servo hose can collapse internally under heavy manifold vacuum during engine overrun. Old vacuum hose can crack due constant flexing as it absorbs engine movement.

[Rotoariesarecool]

Yeah I'd check the hose and the check valve first those are the most common failures. Just buy 2 dollar bit of vacuum hose and test it out to narrow it down

[RRoller123]

sticking check valve sure sounds like this one.

[RenaissanceMan]

Hey again-

So we replaced the booster and the master cylinder with brand new units, and there is still is no boost. Very hard pedal and weak braking power. Vacuum is definitely present, and the vacuum hose is also brand new. Calipers have been rebuilt. At a loss for ideas. Any of you have any other solutions? I know there is some sort of compensation valve for the rear, correct? Could that be contributing?

[kilrwail]

If there's a check valve in the vacuum hose make sure it's not installed backwards. Sounds silly but I've seen it done by a master mechanic.

[RenaissanceMan]

If there's a check valve in the vacuum hose make sure it's not installed backwards. Sounds silly but I've seen it done by a master mechanic.

Thanks for the reply. We did check that, thanks. Check valve is new and oriented in the right direction.

[DieselSpider]

If its all new then the pin length needs to be adjusted. Other than that verify that there actually is good and constant vacuum going to the booster by putting a vacuum gauge on the line. You may simply have a partially blocked vacuum port.

Mine gets its vacuum from a pump mounted on the alternator since its a diesel so I make checking its operation part of my standard checklist. Low intake vacuum means little or no brake boost and since a diesel has no throttle plate there is basically no intake vacuum.

I like to see as close to 21 inches of vacuum at idle as is possible on a gasoline powered engine. The brake booster should be getting its vacuum from below the throttle plates so its getting intake vacuum and not airhorn vacuum which will be much lower.

The front brakes do the bulk of the stopping so a mal-adjusted rear compensator valve would not totally eliminate boost.

What condition is the vacuum reservoir tank in? A leak there or elsewhere in the system could lead to poor engine vacuum especially under a load and then you'd run out of boost after a few concurrent brake applications.

[RenaissanceMan]

Hey again everyone-

So it takes a while between posts for me as I'm helping my father sort this out from a different state over the phone.

Since I last posted, my father went ahead and purchased a new brake booster, new compensator valve, as well as new front calipers, just to eliminate as many variables as possible. Now he is experiencing a pretty strange symptom that I hope someone can shed some light on. When the car is idling, brake function 'feels' normal. Testing with the car in the air on jackstands (with the wheels properly elevated for compensation valve operation) with a helper applying the brakes results in his being able to prevent the wheels from spinning by hand with as much force as he is able to apply. However, as soon as the car is put into gear and engages any bit of momentum, he loses braking power to the point of still being able to turn the wheels by hand with the brakes applied. It is the strangest thing I have ever seen with brakes... Anyone have any clues? Thanks again-

[DieselSpider]

What does your vacuum gauge indicate is going on? If engine vacuum is dropping substantially to the brake booster when you put the engine under load you have either a low vacuum situation from intake/hose leaks, worn engine, valves out of adjustment/timing or potentially the vacuum hose to the brake booster connected to the wrong port on the manifold.

I have a diesel in mine which uses a vacuum pump mounted on the back of the alternator so I do not have the OEM configuration to reference however most carburetors have vacuum ports on them that get their vacuum from either before or after the throttle plates and if someone accidentally connected the brake booster vacuum hose to a port before the throttle plate you could get odd symptoms as you have described. You will only get high vacuum from a port located before the throttle plate when the choke is closed during warmup and then the vacuum level will drop as the choke plate opens and the engine warms up.

Before throwing parts at it a vacuum gauge needs to be put on it to see exactly what your dealing with. If vacuum is dropping into the single numbers then you can run out of brake boost very quickly and the low vacuum situation would need to be addressed first. More is needed than to say vacuum is felt since it needs to be between 15 to 21 inches without a lot of fluttering to have good brake booster operation.

Still even once all vacuum is lost you should be able to press the brake pedal hard enough to stop the car. On a car with a questionable history where you get a hard pedal but limited braking the rubber brake lines would need to be checked. The OEM formulation rubber lines located at each wheel and between the compensator and rear axle may not be compatible with the newer synthetic brake fluids and I have seen them deteriorate within a few months when exposed to it and become almost totally plugged by internal swelling or shredding of the liner even though there were no externally visible signs of this. When the liner in the brake lines start to shred they can become like one way valves that will either prevent the brakes from being applied or allow them to be applied but not released.

Dot 3 brake fluid should be used on old systems where there is a potential that old formulation rubber components that will swell when exposed to the new synthetic fluids may be present.

[RenaissanceMan]

This is really great info, thanks so much for the insight. Going to share this with my father and see if we can further diagnose!

What does your vacuum gauge indicate is going on? If engine vacuum is dropping substantially to the brake booster when you put the engine under load you have either a low vacuum situation from intake/hose leaks, worn engine, valves out of adjustment/timing or potentially the vacuum hose to the brake booster connected to the wrong port on the manifold.

I have a diesel in mine which uses a vacuum pump mounted on the back of the alternator so I do not have the OEM configuration to reference however most carburetors have vacuum ports on them that get their vacuum from either before or after the throttle plates and if someone accidentally connected the brake booster vacuum hose to a port before the throttle plate you could get odd symptoms as you have described. You will only get high vacuum from a port located before the throttle plate when the choke is closed during warmup and then the vacuum level will drop as the choke plate opens and the engine warms up.

Before throwing parts at it a vacuum gauge needs to be put on it to see exactly what your dealing with. If vacuum is dropping into the single numbers then you can run out of brake boost very quickly and the low vacuum situation would need to be addressed first. More is needed than to say vacuum is felt since it needs to be between 15 to 21 inches without a lot of fluttering to have good brake booster operation.

Still even once all vacuum is lost you should be able to press the brake pedal hard enough to stop the car. On a car with a questionable history where you get a hard pedal but limited braking the rubber brake lines would need to be checked. The OEM formulation rubber lines located at each wheel and between the compensator and rear axle may not be compatible with the newer synthetic brake fluids and I have seen them deteriorate within a few months when exposed to it and become almost totally plugged by internal swelling or shredding of the liner even though there were no externally visible signs of this. When the liner in the brake lines start to shred they can become like one way valves that will either prevent the brakes from being applied or allow them to be applied but not released.

Dot 3 brake fluid should be used on old systems where there is a potential that old formulation rubber components that will swell when exposed to the new synthetic fluids may be present.