Brake and idle problems at elevation of 6,000 ft.

[Bob King]

Gentlemen - Here is a good one for you. Last August on my way from Sacramento (elevation 140' above sea level) to Reno for Hot August Nights, I stopped off at Truckee (elevation about 6,500') and the brake pedal went to the floor. I still had a brake at the bottom of the pedal and the engine barely ran at about 200 - 300 rpm. I went on down to Reno and returned home driving carefully.

When I got home I checked the vacuum hose from the engine to power brake booster and tighten up the clamps at each end. Then with the engine off, I pumped the brake pedal and listened for air whoosing out of the booster which happened then and happens now. So I figured it was not the booster and went to work and replaced the master cylinder, front caliper seals and the 2 front rubber hose brake lines, and bled the heck out of the front and back calipers. The front right caliper was leaking before the trip to Reno, so I thought maybe air was leaking back into the caliper and lines. So, before yesterday, the brakes were working pretty well and there are no brake fluid leaks.

Yesterday I took the car up to ski Alpine Meadows, with the skis through the
back window, and when I stopped off at Kingvale,(6,500' elevation), the brake pedal went to the floor, but with braking, and the engine barely ran at 200-300 rpm. Drove on to Alpine, skied, drove back down the hill, and stopped at about 180' elevation and the engine idle returned to normal at 800rpm, (cam is about 490 degrees lift with engine vacuum at idle of, I believe, about 11). The brake pedal went to the floor with braking at the bottom of the pedal.

I suspect there maybe 2 different problems. This morning I pulled off the vacuum check valve between the intake mainifold and the 2 lines (one line to the reserve tank and the other to under the dash) and was able to blow air from the double post side of the valve to the single post side, so I think this valve is bad. But I'm at a loss right now about the brakes, other than it must be the booster.

Pretty good one, huh? But I'm sure you've heard it all before.

Thakks for giving me your good and best thoughts. Bob King

[Jack H.]

Re: Brake and idle problems at elevation of 6,000

This can be a side-effect of using silicone based brake fluid--it's not hydroscopic (affinity to absorb water) but it is aeroscopic....

[Bob King]

Re: Brake and idle problems at elevation of 6,000

Jack - Thanks for the response, but you left me kind of hanging. Should I change to the DOT 5 fluid, and if so, what other problems might I encounter with my 1969 brake lines if I change to DOT 5?

It seems odd to me that I have a brake and an idle problem at elevation. And I don't remember back in the 60's and 70's having the "elevation" brake problem, when I had 60's and 70's Chevys and Olds, driving from Kansas to Buena Vista or Vail in Colorado. The passes there, as you know, are 10,000 feet.

Can you advise me further. Thanks. Bob KIng

[Michael W.]

Re: Brake and idle problems at elevation of 6,000

A bad booster or vacuum check valve wouldn't cause your pedal to go to the floor. A bad booster would give you a hard, non-power brake feeling. Your check valve is supposed to allow air from the the two post side to the one post side. Try blowing on the one post side, there should be no flow. Even if found bad, this should not contribute to the abnormal idle issue. A bad check valve usually causes the headlight doors and/or wiper door to open and close at the wrong time.

You might have lead yourself down the wrong path by thinking that the altitude has something to do with it.

Have you recently gone on a drive of similar DISTANCE but with no variation in altitude? Did the brake pedal remain firm? If no, then you may have a problem with lateral runout on one or more of your brake rotors. This runout tends to 'knock' the caliper pistons back in their bores (giving a low pedal) and also allows some air to sucked around the piston seals into the calipers, giving a spongy pedal.

Try measuring the runout on your rotors, if over .005" it's a probable contributor.

[Joe L.]

Re: Brake and idle problems at elevation of 6,000

Bob------

The high altitude brake problem occurs with BOTH DOT 3 and 4 glycol fluid and DOT 5 silicone fluid. I've used both in my car and have had exactly the same problem with either. In fact, I switched to DOT 5 partially for the reason of seeing if it would help eliminate the problem. It didn't. The problem got no better nor did it get any worse using DOT 5. I've also known others that never, ever used DOT 5 and had the same problem. At the same time, I know of folks that live at very high altitude locations (above 6,000 MSL), use DOT-5 in their C2/C3 Corvettes and have no problems, whatsoever, with a soft pedal.

I have tried a PLETHORA of things to solve this problem (every one you can think of and then some) and not a single one has been effective, at all. I am now absolutely convinced that the problem is caused by air trapped somewhere in the system, but I've never been able to get it out. At altitude, the air pocket expands due to the lower atmospheric air pressure and creates the "soft pedal".

When my car is back on the road again, I'll be back at it again.

[Bob King]

Re: Brake and idle problems at elevation of 6,000

Mike - Thank you for your info. I drive the 69' Vette all over Roseville and Sacramento (elevation 160') and have had a good brake pedal.

Also I blew and sucked into the single post of the vacuum check valve and air goes both ways through this valve, so I guess it needs replacement.

I just read Joe Luca's response about altitude and air in the brake lines and that seems to make sense to me; what do you think?

But I am still puzzled by the very low idle at altitude; there must be a vacuum leak somewhere. Got any other ideas for me.

Thanks so much. Bob King

[Bob King]

Re: Brake and idle problems at elevation of 6,000

Joe - Thanks for the response about air in the brake lines; which makes sense to me. Maybe I should go up to Donner Pass, take my jack, and bleed the heck out the the brakes up there.

Do you have any thoughts about the low rpm idle, at altitude, and a probable vacuum leak somewhere in the system? My reserve tank holds a vacumm for 10-15 seconds; could that be the problem?

Thanks for your help. Bob King

[Jean C.]

Jack...brake problems at 9,000 feet

So Jack, after driving from Houston to Breck in my '64 with silicone fluid for the Regional in June, I should expect to make use of the runaway truck ramps on I-70 Eastbound when headed home?? I'll let you run interference for me on the downhill side. We got us a convoy!

Charlie Cadenhead
#40193

[Bob King]

Re: Jack...brake problems at 9,000 feet

Charlie - Don't worry, westbound I-80 in the Sierra has probably 3 truck runaway ramps and I never had to use them on either of my trips; but you have to be careful and not drive like you're in a Vette. Bob King

[Clem Z.]

Re: Brake and idle problems at elevation of 6,000

you need to reduce the jet area 2% for every 1500 ft above sea level and i a sure the idle circut would need to be leaned out also

[Duke W.]

Re: Brake and idle problems at elevation of 6,000

Loss of idle speed at altitude is normal for a carbureted engine. If you plan on spending a lot of time at altitude, set you normal altitude idle 100-200 RPM higher.

Duke

[Joe L.]

Re: Brake and idle problems at elevation of 6,000

Bob-----

What you've suggested as far as brake bleeding is one of the things that I've already tried. It didn't work. I tried bleeding the brakes TWICE in the parking lot of the hotel I stayed at at Mammoth Lakes, CA (elevation 7,300 MSL). I could not get a hard pedal no matter how much I bled the system. Like I say, I've tried everything that you could think of and then some.
You name it, I'll bet that I've tried it.

As far as the idle goes, as other have said, with a carbureted engine with no atmospheric pressure compensation, you WILL have a poor idle at high altitude. For one thing, the engine is running extremely rich due to the much lower density of oxygen in the atmosphere. Some sophisticated carburetors (like the Keizen variety used on many Honda engines prior to FI being used across the board) had an atmospheric pressure compensation system. So, these would run well at higher altitudes. Corvette carburetors never had any such compensation, though.

[Clem Z.]

Re: Brake and idle problems at elevation of 6,000

if you have a holley carb you can convert it to screw in idle and main air bleeds and change them for your high altitude trips. this done externally as they are located just above the venturies.

[Bob King]

Re: Brake and idle problems at elevation of 6,000

Joe - Thanks alot for your responses.

Donner Summit is 7,250' or so; if I thought it would work I would drive up there and try it. My front right caliper did not totally bleed out the last time, so at about 5:00 today I bleed it for a while and still could not get all of the air our. That maybe the one causing the problem. I drove it out on Rocky Ridge and Douglas and the brake pedal was OK, close to what it was before yesterday. When I replaced the caliper piston seals on the front right, I also removed the rotor and re-packed the bearings and eyeball checked for run out, and I could not see any wobble; I thought this rotor turned very true. I also have unplugged the electric connection on the proportioning valve (right below the master cylinder), since after driving for awhile the brake light would come on. So maybe one of the other rotors is bad.

The front calipers have new rubber brake lines ($25 each at the Northgate NAPA store, in case you know Sacramento). I remember bleeding the brakes those many years ago on my 71' Cutlass and 64' Malibu, and I could always get the air out. So I'm puzzled why the front right will not bleed. Maybe I'll remove it again and see what I can find. Maybe tomorrow after work I will bleed the rear calipers, to see what is going on there, first.

When I get this brake problem fixed (hopefully), next time I head up the hill, I'll adjust the idle speed to 1,100rpm. I remember now that I did that in Reno last August.

Thanks for your thoughts. Bob King

[Bob King]

Re: Brake and idle problems at elevation of 6,000

Clem - I have a new Edelbrock 1406, (I think that's the number) on the motor now. I also have a Holley 4175, but with that carb. I can't use my original air cleaner base comfortably and also I blew out a power valve. I also have a 71' Rochester that was on the car when I purchased it 2.5 years ago. When I had the motor re-built last year, the Rochester could not handle the lower engine vacuum of around 11 HG.

So, the Kragen Speed shop guy made me a good deal on the Edelbrock and it runs great (except for the high elevation idle problem). I remember when I got to Reno last summer I adjusted the idle speed up a few 100 rpm's and that worked OK for the Edelbrock. Got any ideas on other high altitude adjustments for the Edlebrock?

Thanks alot for your help. Bob King