Brake dilemma, need advice

[SCOTI]

I recall reading somewhere that a minimum of 17" vacuum is needed for full brake pressure. Although the 14" sounds close when the motor is reving, you won't have rpm's up when the brake pedal is applied.

[Painter D]

Either way you look at it shouldn't I be able to push the pedal hard enough that at some point they lock up? Even if I had no power brakes at all? If I have to I'll just spend the bread and get a hydroboost setup if I can't get enough out of my current setup.



BTW SCOTI thanks for telling me about Entropy ,I ordered some elec. fans and a nice aluminum shroud from them today.

[SCOTI]

Originally Posted by 69chevyshort

Either way you look at it shouldn't I be able to push the pedal hard enough that at some point they lock up? Even if I had no power brakes at all? If I have to I'll just spend the bread and get a hydroboost setup if I can't get enough out of my current setup.



BTW SCOTI thanks for telling me about Entropy ,I ordered some elec. fans and a nice aluminum shroud from them today.

Glad you found what you were looking for.

As for the pressure issue, I would think they should still lock up but it may not be the case (a pwr steering set-up w/o the pwr assist is more difficult to steer vs. a manual steer set-up). My guess is a manual brake set-up has the internal valving calculated for that specific design requirement as does a power brake master cylinder. Take away the 'boost' assist from the system, & it may be more difficult than we think to get them to lock-up.

[Painter D]

HHMMM, interesting you may be on to something there. Well where do I go from here hydroboost ,or should I wait until I get my new engine (probably a 502) and see if that motor makes enough vacuum?


Decisions, decisions.

[SCOTI]

Originally Posted by 69chevyshort

HHMMM, interesting you may be on to something there. Well where do I go from here hydroboost ,or should I wait until I get my new engine (probably a 502) and see if that motor makes enough vacuum?


Decisions, decisions.

You could also look into an electric vacuum pump. GM used them on Caddys & such plus they're available via the aftermarket for applications where consistent vacuuum is an issue.

[skoffice]

Here's a good way to check your vacuum. Hook up the vacuum gauge at the engine. Now, adjust your timing until the gauge reads the highest reading. Next, back your timing off 2 degrees. This will show you two things, the highest amount of vacuum your engine is capable of producing, and you will be shockingly close to the correct timing for your engine. Gone by this method a few times before and it works perfectly. Also a couple things, have you thought about a residual valve? If it's an aftermarket MC its common for them to not have it built in. Also, fire up the engine in the evening sometime, then come out the next day and pull the check valve. There should be vacuum still holding in the booster even overnight. Also, I had one of those vacuum pumps, it worked alright but it was very noisy

[Longhorn321]

Originally Posted by 69chevyshort

Either way you look at it shouldn't I be able to push the pedal hard enough that at some point they lock up? Even if I had no power brakes at all?

Yes, you should.

[Painter D]

Originally Posted by skoffice

Here's a good way to check your vacuum. Hook up the vacuum gauge at the engine. Now, adjust your timing until the gauge reads the highest reading. Next, back your timing off 2 degrees. This will show you two things, the highest amount of vacuum your engine is capable of producing, and you will be shockingly close to the correct timing for your engine. Gone by this method a few times before and it works perfectly. Also a couple things, have you thought about a residual valve? If it's an aftermarket MC its common for them to not have it built in. Also, fire up the engine in the evening sometime, then come out the next day and pull the check valve. There should be vacuum still holding in the booster even overnight. Also, I had one of those vacuum pumps, it worked alright but it was very noisy

Dude ,don't even get me started on timing. The engine in my truck is the only engine in the world that refuses to have the timing set correctly. Amongst other engine issues timing is the absolute last thing I want to mess with on my truck. That's (one of the many reasons) I'm ditching this engine and getting a new one.


I think I'm just gonna wait a little longer and get a new Wilwood MC and get rear disc brakes. If that doesn't do it I'll invest in a hydroboost setup.

[skoffice]

Sounds like the problem your having is... You need more horsepower!!!!

[Painter D]

Thinking back, my brakes were like this before I had a big cam and my motor was essentially stock. I don't know what the vacuum readings were then but they should have been alot better than they are now. Which would only leave the MC and proportioning valve to check into.


Is there any way I can test those two things out?

[skoffice]

If the master cylinder is bad you should be able to tell by bench bleeding it again. It may show air bubbles from a leaky seal or something. The proportioning valve I think you'd just have to replace to find out..

[Longhorn321]

Originally Posted by Longhorn321

Try this method of bleeding:
1. Open at least one if not both front bleeders and put a vacuume hose from the bleeder into a plastic drink bottle (to keep from making a mess).
2. While the front/s are open, bleed the RR and then the LR. Keep checking the master cylinder to make sure you don't run dry.
3. If you have the LF open, close it.
4. Bleed the RF.
5. Bleed the LF.
If your pedal is not rock hard after trying this, try it again at least once. If still not rock hard, replace the master cylinder. FYI, even if your booster is bad, if you stand on the brakes hard enough, you should be able to lock the brakes.

Did you try bleeding it like this? My dad who has forgotten more about cars than I will ever know (I am a technical instructor for BMW) told me that these old trucks have something wacky in the M/C that will not let the rears bleed very easily if you don't have the fronts or at least one of the front bleeders open. Too much pressure in the cylinder wont let the piston bottom out in the rear portion? Or it bottoms out too early in the front portion? I don't think even he remembers exactly why it's wacky...it just is and you will have a lot better luck with the fronts or at least one front bleeder open when you do the rears.

[70jnktrk]

http://www.markwilliams.com/braketech.aspx
good luck with the 'problem' ..thought this might shed some light on the subject..

[Painter D]

Originally Posted by 70jnktrk

http://www.markwilliams.com/braketech.aspx
good luck with the 'problem' ..thought this might shed some light on the subject..

THANKS for the link.



Update: Upon closer examination of my brake system with the help of board member SCOTI over the phone I may have found the culprit of the problem. Years ago when I plumbed the system I may have used too small of a brake line for the front after all that one line feeds two calipers so it probably doesn't need to be undersized. I'm going to order some nice stainless steel lines and try that and see if that fixes the problem.

[lolife99]

What are you calling too small?
Smaller than 3/16" or 1/4" brake line?

[SCOTI]

Originally Posted by lolife99

What are you calling too small?
Smaller than 3/16" or 1/4" brake line?

He's using factory drum lines on a disc system. I suggested getting a front line kit for a 71-72 & go from there....

[lolife99]

Originally Posted by SCOTI

He's using factory drum lines on a disc system. I suggested getting a front line kit for a 71-72 & go from there....

That's what I have on my '69. But I have manual disc brakes. No booster.

[DKN]

The line size will not change fluid pressure. It will change the reaction time of the caliper by a few milliseconds. You will need special lab equipment to find the exact time vs. pressure. Your primary problem seams to be a lack of pressure. A smaller bore master cylinder will help you get more pressure, but you will get more pedal travel, the pedal may go all the way to the floor. More vacuum for you booster (assuming the booster is working right) or a hydra boost will give you more pressure, and you will not have to bleed the system again.

FYI: Inside the factory proportioning valve the smallest passage is about 0.06” in diameter or 0.003sq in. The ID if a 3/16” brake line is 0.1285 in diameter or 0.013sq in. The proportioning valve is about 4-1/2 times smaller than the brake line. You can change the brake lines, but it will not make more pressure and you will not be able to stop any better.

Danny Nix
CPP

[Painter D]

Originally Posted by DKN

The line size will not change fluid pressure. It will change the reaction time of the caliper by a few milliseconds. You will need special lab equipment to find the exact time vs. pressure. Your primary problem seams to be a lack of pressure. A smaller bore master cylinder will help you get more pressure, but you will get more pedal travel, the pedal may go all the way to the floor. More vacuum for you booster (assuming the booster is working right) or a hydra boost will give you more pressure, and you will not have to bleed the system again.

FYI: Inside the factory proportioning valve the smallest passage is about 0.06” in diameter or 0.003sq in. The ID if a 3/16” brake line is 0.1285 in diameter or 0.013sq in. The proportioning valve is about 4-1/2 times smaller than the brake line. You can change the brake lines, but it will not make more pressure and you will not be able to stop any better.

Danny Nix
CPP

Well good grief, no offense but I hope you're wrong. I guess I'll just keep buying parts until something fixes it.

[SCOTI]

Originally Posted by 69chevyshort

Well good grief, no offense but I hope you're wrong. I guess I'll just keep buying parts until something fixes it.

The reason I felt it was a good move is you get the 'correct' lines & the 'kits' typically come w/a new prop valve as well which will help eliminate multiple variables.

[Painter D]

Originally Posted by SCOTI

The reason I felt it was a good move is you get the 'correct' lines & the 'kits' typically come w/a new prop valve as well which will help eliminate multiple variables.

That's what I'm going to do......................if that don't fix it I'll just keep throwing money at it until it does.

[Painter D]

I thought I'd post some pics of what I'm dealing with so that way if anyone "physically sees" a problem they can speak up and tell me what I'm missing. Notice the size of the lines coming out of each one of the reservoirs on the MC, the forwardmost reservoir is for the front brakes and rear resorvoir is for the rear brakes. I find this to be bassackwards compared to most vehicles I've looked at ,but that's the way the directions said to plumb it. Usually the larger reservoir of the two is for the front brakes but on my MC both reservoirs are the same size. The bore size measures 1.195" (if I measured it right) so if I had to guess that would be 1 1/8" bore size. A mechanic at work (Ford dealership) suggested I try swapping the front and rear lines and see if that makes a diffeence. Anybody else got any suggestions?




BTW I'm fully aware my MC and engine compartment for that matter look like chit. I'm in the process of redoing and replacing everything under the hood.

[KQQL IT]

My factory prop. valve was stuck and wouldnt work. I had to take the prop. valve apart and manually fix it. Shouldve replaced it tho

[lolife99]

How were the brakes before you intsalled the adjustable porportioning valve?

[Painter D]

Originally Posted by lolife99

How were the brakes before you intsalled the adjustable porportioning valve?

They were worse. The rears would lock up really easy.