I was looking for anything which may cause my brake pedal to hold a bit lower than I would like. First thought was to check for a possible problem with the pushrod length, between brake booster and master cylinder. My approach to testing was to open the bleeders on the master, one at a time, watch the column of fluid as I slowly press on the brake pedal.

Strangely, when I slowly press on the brake pedal, I see the column of fluid go down initially, then go back up and keep climbing as I push the pedal further. The brief drop seen just after I begin pushing down on the brake pedal mystifies me. I expected the column of fluid to remain steady, begin to climb once pressure began to build. Is this normal?

Sorry the video appears rotated 90 degrees...
master_cylinder.MOV

 

*****PROFESSIONAL OPINION*****

I do not know why you are doing what you are doing in this test. What it shows me is that the master cylinder is moving fluid..as it should....but I can only assume that the clear tube you have fluid going up into is attached to the front brake line port on the master cylinder itself.

Knowing that a fluid cannot be compressed...I am curious on how bad off your brake pedal actually is.

If it 'feels' that is it going lower than normal...did you do anything to your brake system recently???

You wrote:
I was looking for anything which may cause my brake pedal to hold a bit lower than I would like.

When did this 'problem' become noticeable??? Out of the blue or over time and kinda crept up on you.

I can type and type and so on but without knowing the particulars.,..I might type out stuff that does not apply. Such as ...has the car not been driven for along time??

DUB

 

Hi DUB,

The hose is connected to the bleeder on the master cylinder.
This car originally came with manual brakes, converted to power brakes many years ago. The brake pedal hasn't felt right for a long time. A local shop replaced the push rod some time ago along with the master cylinder. Could be when I changed over to power brakes I had the wrong pushrod? Just a guess.

I replaced the master cylinder again a few years back when I discovered it was leaking into the booster. Haven't had what I feel is a firm pedal with minimal movement since then. It's not bad right now, just a bit more travel than I'm used to when driving my every day cars. The brakes seem to hold evenly, tested it today in a parking lot, braking, let the wheel go to see if it veered one way or another.

My test was to see if I got an immediate reaction when lightly pushing on the pedal. Seeing fluid movement with little pedal movement suggests to me the pushrod length is probably OK.

The car hasn't been driven much at all this season. Two or three short drives since spring, most recent today.

All calipers have been sleeved, rears rebuilt with new lip seals a few years ago. No leaks which I am aware of, may need to check again. Last I checked the rear rotor run out was OK, could be a bit better but would need to be shimmed. It was in spec, decided to leave it alone.

I am using dot 5 fluid which some say contributes to a mushy pedal. It's been so long since I've had dot 3 fluid I can't say the fluid is the culprit for sure.

One other piece of information. I replaced the lines going from the junction block to the rear, all lines in the rear, with stainless. No leaks but did take some time to bleed. I've wondered if air could be caught in the bend where the line goes up, then bends back to join the junction block on the top (proportioning valve?) ? I've bled the brakes many times with a power bleeder. Tried gravity bleed as well.

 

the pushrod for power brakes and manual brakes use different holes in the brake pedal as well as different bracket and striker for the stop light switch

 

OH....so it was a manual brake car from the factory. That can explain a lot right there. I have been burned by this conversion and found a problem with a 1972 Corvette I did once when I installed a power booster/master cylinder combo.

From the day you had the power brakes installed ...did they ever feel really good????

I think I know what is wrong ( or at least what I found that worked for me)...but I also need to know are you the one actually going to do this work or are you giving it to someone???

DUB

 

I do all work myself unless it involves welding, bodywork, or sometimes something I'm not getting anywhere with. Generally prefer not to send it out, most often I get a large bill.

The brakes have never been stellar IMO. My standards have changed considerably as I get older, less tolerant to things which are a bit off.


I did the conversion back in mid 80's. Went by the AIM, know what parts were different. What I'm not sure of is where the booster came from, bought in a swap meet. Looks correct to me. It's been a long time, don't recall the details of what was changed but think I remember having to move the hole on the firewall downward. Did some work to fill part of the original hole so the conversion wouldn't be obvious.

Anything you can offer would be appreciated. I'll take a look.

Strange that a local shop just went over the car, gave me a summary of what it needs before I should even begin to thinking about painting. Odd how the brakes weren't mentioned.

 

Thanks. The conversion was done long ago, pretty sure I moved the link to the proper hole in the pedal but can check to be sure.

The pushrod I refer to is the one between the booster and master cylinder. Seems when I did the conversion I ended up with one which was a bit short, causing the pedal to travel further before engaging the master cylinder.

 

on a power brake car the pushrod clevis pin goes in the lower hole of the brake pedal...

 

The short pin is not a big problem due to that is what is used in the later design booster...but what I can concerned about is what the back of the master cylinder looks like.

If you have the short pin on your booster....the center dimple in your master cylinder should look like it was barely counter sunk...and NOT a hole with any depth to it.

PM sent.

DUB

 

The brake pedal rests about 5" from the carpet to the top surface of the pedal. Difficult to get an accurate measurement without a solid reference point, carpet and padding underneath. When I press on the pedal by hand it starts to become firm around 3" from carpet to top of the pedal, 2" travel. The AIM spec calls for 1.8" with 80 lbs force. Seems I have a bit more travel than spec?

Going through all possibilities I can think of, I'm wondering if the height from floor to the pedal at rest is correct. I've read somewhere there are two boosters, one has a shorter shaft connecting to the pedal. My brake pedal height at rest is similar to the gas pedal. Clutch pedal is much higher.

It appears I have a long push-rod coming out of the booster. Hole in the back of the master is about 1.5" deep, rough measurement.

Pedal attachment point looks right, no lower hole. The position of the booster leaves the shaft pretty much pointing to the proper hole.

 

You wrote:
It appears I have a long push-rod coming out of the booster. Hole in the back of the master is about 1.5" deep, rough measurement.

A very slight adjustment on this bolt on the shaft can equate to changing the dynamic of your pedal travel.

As I wrote in the PM that was sent..I am careful when I am measuring this and I do it precisely and I check it.

I do not know if I would loose any sleep over the measurement in the book being 2".... and you are at 1.80". I would have to bee there and feel the pedal and 'see' it for myself and if I did not like it I would go and check the bolt on the shaft as previously mentioned.

In my mind..there is absolutely no way that each and every master cylinder that is made from every manufacturer that makes them...makes them all with the EXACT same depth on the valving where the shaft and bolt contact.

You might be surprised on what a 1/4 of a turn out on that bolt can do. But still considering the one inch of travel that the shaft should not exceed... As another rule of thumb.

DUB